diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index b6e72327004..e6b0c275c11 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -148,6 +148,9 @@ Bug Fixes
   work as intended for an alias spanning more than 1 collection. 
   (Thomas Egense, Shawn Heisey, Mark Miller)
 
+* SOLR-5418: Background merge after field removed from solr.xml causes error. 
+  (Reported on user's list, Robert M's patch via Erick Erickson)
+
 Optimizations
 ----------------------  
 
diff --git a/solr/core/src/java/org/apache/solr/core/SchemaCodecFactory.java b/solr/core/src/java/org/apache/solr/core/SchemaCodecFactory.java
index 7673131faa2..088375d00ac 100644
--- a/solr/core/src/java/org/apache/solr/core/SchemaCodecFactory.java
+++ b/solr/core/src/java/org/apache/solr/core/SchemaCodecFactory.java
@@ -54,25 +54,23 @@ public class SchemaCodecFactory extends CodecFactory implements SolrCoreAware {
     codec = new Lucene46Codec() {
       @Override
       public PostingsFormat getPostingsFormatForField(String field) {
-        final SchemaField fieldOrNull = core.getLatestSchema().getFieldOrNull(field);
-        if (fieldOrNull == null) {
-          throw new IllegalArgumentException("no such field " + field);
-        }
-        String postingsFormatName = fieldOrNull.getType().getPostingsFormat();
-        if (postingsFormatName != null) {
-          return PostingsFormat.forName(postingsFormatName);
+        final SchemaField schemaField = core.getLatestSchema().getFieldOrNull(field);
+        if (schemaField != null) {
+          String postingsFormatName = schemaField.getType().getPostingsFormat();
+          if (postingsFormatName != null) {
+            return PostingsFormat.forName(postingsFormatName);
+          }
         }
         return super.getPostingsFormatForField(field);
       }
       @Override
       public DocValuesFormat getDocValuesFormatForField(String field) {
-        final SchemaField fieldOrNull = core.getLatestSchema().getFieldOrNull(field);
-        if (fieldOrNull == null) {
-          throw new IllegalArgumentException("no such field " + field);
-        }
-        String docValuesFormatName = fieldOrNull.getType().getDocValuesFormat();
-        if (docValuesFormatName != null) {
-          return DocValuesFormat.forName(docValuesFormatName);
+        final SchemaField schemaField = core.getLatestSchema().getFieldOrNull(field);
+        if (schemaField != null) {
+          String docValuesFormatName = schemaField.getType().getDocValuesFormat();
+          if (docValuesFormatName != null) {
+            return DocValuesFormat.forName(docValuesFormatName);
+          }
         }
         return super.getDocValuesFormatForField(field);
       }
diff --git a/solr/core/src/test-files/solr/collection1/conf/solrconfig-minimal.xml b/solr/core/src/test-files/solr/collection1/conf/solrconfig-minimal.xml
index 78a4eb711d3..26af3a7cf5b 100644
--- a/solr/core/src/test-files/solr/collection1/conf/solrconfig-minimal.xml
+++ b/solr/core/src/test-files/solr/collection1/conf/solrconfig-minimal.xml
@@ -29,7 +29,7 @@
                     class="${solr.directoryFactory:solr.NRTCachingDirectoryFactory}"/>
 
   <xi:include href="./solrconfig.snippet.randomindexconfig.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/>
-
+  <codecFactory class="solr.SchemaCodecFactory"/>
   <jmx/>
   <updateHandler class="solr.DirectUpdateHandler2">
     <!--updateLog>
diff --git a/solr/core/src/test/org/apache/solr/core/TestCodecSupport.java b/solr/core/src/test/org/apache/solr/core/TestCodecSupport.java
index c970652921b..c462eb9bb12 100644
--- a/solr/core/src/test/org/apache/solr/core/TestCodecSupport.java
+++ b/solr/core/src/test/org/apache/solr/core/TestCodecSupport.java
@@ -83,16 +83,4 @@ public class TestCodecSupport extends SolrTestCaseJ4 {
     assertEquals("Lucene45", format.getDocValuesFormatForField("foo_memory").getName());
     assertEquals("Lucene45", format.getDocValuesFormatForField("bar_memory").getName());
   }
-
-  public void testUnknownField() {
-    Codec codec = h.getCore().getCodec();
-    PerFieldPostingsFormat format = (PerFieldPostingsFormat) codec.postingsFormat();
-    try {
-      format.getPostingsFormatForField("notexisting");
-      fail("field is not existing");
-    } catch (IllegalArgumentException e) {
-      //
-    }
-
-  }
 }
diff --git a/solr/core/src/test/org/apache/solr/schema/ChangedSchemaMergeTest.java b/solr/core/src/test/org/apache/solr/schema/ChangedSchemaMergeTest.java
new file mode 100644
index 00000000000..56247c1f5f6
--- /dev/null
+++ b/solr/core/src/test/org/apache/solr/schema/ChangedSchemaMergeTest.java
@@ -0,0 +1,148 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.solr.schema;
+
+import org.apache.commons.codec.Charsets;
+import org.apache.commons.io.FileUtils;
+import org.apache.solr.SolrTestCaseJ4;
+import org.apache.solr.common.util.NamedList;
+import org.apache.solr.core.CoreContainer;
+import org.apache.solr.core.SolrCore;
+import org.apache.solr.request.LocalSolrQueryRequest;
+import org.apache.solr.request.SolrQueryRequest;
+import org.apache.solr.update.AddUpdateCommand;
+import org.apache.solr.update.CommitUpdateCommand;
+import org.apache.solr.update.UpdateHandler;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.io.File;
+import java.io.IOException;
+
+public class ChangedSchemaMergeTest extends SolrTestCaseJ4 {
+  @BeforeClass
+  public static void beforeClass() throws Exception {
+    initCore();
+  }
+
+  private final File solrHomeDirectory = new File(TEMP_DIR, getSimpleClassName());
+  private File schemaFile = null;
+
+  private void addDoc(SolrCore core, String... fieldValues) throws IOException {
+    UpdateHandler updater = core.getUpdateHandler();
+    AddUpdateCommand cmd = new AddUpdateCommand(new LocalSolrQueryRequest(core, new NamedList<Object>()));
+    cmd.solrDoc = sdoc((Object[]) fieldValues);
+    updater.addDoc(cmd);
+  }
+
+  private CoreContainer init() throws Exception {
+
+    if (solrHomeDirectory.exists()) {
+      FileUtils.deleteDirectory(solrHomeDirectory);
+    }
+    assertTrue("Failed to mkdirs workDir", solrHomeDirectory.mkdirs());
+    File changed = new File(solrHomeDirectory, "changed");
+    copyMinConf(changed, "name=changed");
+    // Overlay with my local schema
+    schemaFile = new File(new File(changed, "conf"), "schema.xml");
+    FileUtils.writeStringToFile(schemaFile, withWhich, Charsets.UTF_8.toString());
+
+    String discoveryXml = "<solr></solr>";
+    File solrXml = new File(solrHomeDirectory, "solr.xml");
+    FileUtils.write(solrXml, discoveryXml, Charsets.UTF_8.toString());
+
+    final CoreContainer cores = new CoreContainer(solrHomeDirectory.getAbsolutePath());
+    cores.load();
+    return cores;
+  }
+
+  @Test
+  public void testOptimizeDiffSchemas() throws Exception {
+    // load up a core (why not put it on disk?)
+    CoreContainer cc = init();
+    SolrCore changed = cc.getCore("changed");
+    try {
+
+      // add some documents
+      addDoc(changed, "id", "1", "which", "15", "text", "some stuff with which");
+      addDoc(changed, "id", "2", "which", "15", "text", "some stuff with which");
+      addDoc(changed, "id", "3", "which", "15", "text", "some stuff with which");
+      addDoc(changed, "id", "4", "which", "15", "text", "some stuff with which");
+      SolrQueryRequest req = new LocalSolrQueryRequest(changed, new NamedList<Object>());
+      changed.getUpdateHandler().commit(new CommitUpdateCommand(req, false));
+
+      // write the new schema out and make it current
+      FileUtils.writeStringToFile(schemaFile, withoutWhich, Charsets.UTF_8.toString());
+
+      IndexSchema iSchema = IndexSchemaFactory.buildIndexSchema("schema.xml", changed.getSolrConfig());
+      changed.setLatestSchema(iSchema);
+
+      addDoc(changed, "id", "1", "text", "some stuff without which");
+      addDoc(changed, "id", "5", "text", "some stuff without which");
+
+      changed.getUpdateHandler().commit(new CommitUpdateCommand(req, false));
+      changed.getUpdateHandler().commit(new CommitUpdateCommand(req, true));
+    } finally {
+      if (changed != null) changed.close();
+      if (cc != null) cc.shutdown();
+    }
+  }
+
+
+  private static String withWhich = "<schema name=\"tiny\" version=\"1.1\">\n" +
+      "  <fields>\n" +
+      "    <field name=\"id\" type=\"string\" indexed=\"true\" stored=\"true\" required=\"true\"/>\n" +
+      "    <field name=\"text\" type=\"text\" indexed=\"true\" stored=\"true\"/>\n" +
+      "    <field name=\"which\" type=\"int\" indexed=\"true\" stored=\"true\"/>\n" +
+      "  </fields>\n" +
+      "  <uniqueKey>id</uniqueKey>\n" +
+      "\n" +
+      "  <types>\n" +
+      "    <fieldtype name=\"text\" class=\"solr.TextField\">\n" +
+      "      <analyzer>\n" +
+      "        <tokenizer class=\"solr.WhitespaceTokenizerFactory\"/>\n" +
+      "        <filter class=\"solr.LowerCaseFilterFactory\"/>\n" +
+      "      </analyzer>\n" +
+
+      "    </fieldtype>\n" +
+      "    <fieldType name=\"string\" class=\"solr.StrField\"/>\n" +
+      "    <fieldType name=\"int\" class=\"solr.TrieIntField\" precisionStep=\"0\" positionIncrementGap=\"0\"/>" +
+      "  </types>\n" +
+      "</schema>";
+
+  private static String withoutWhich = "<schema name=\"tiny\" version=\"1.1\">\n" +
+      "  <fields>\n" +
+      "    <field name=\"id\" type=\"string\" indexed=\"true\" stored=\"true\" required=\"true\"/>\n" +
+      "    <field name=\"text\" type=\"text\" indexed=\"true\" stored=\"true\"/>\n" +
+      "  </fields>\n" +
+      "  <uniqueKey>id</uniqueKey>\n" +
+      "\n" +
+      "  <types>\n" +
+      "    <fieldtype name=\"text\" class=\"solr.TextField\">\n" +
+      "      <analyzer>\n" +
+      "        <tokenizer class=\"solr.WhitespaceTokenizerFactory\"/>\n" +
+      "        <filter class=\"solr.LowerCaseFilterFactory\"/>\n" +
+      "      </analyzer>\n" +
+      "    </fieldtype>\n" +
+      "    <fieldType name=\"string\" class=\"solr.StrField\"/>\n" +
+      "    <fieldType name=\"int\" class=\"solr.TrieIntField\" precisionStep=\"0\" positionIncrementGap=\"0\"/>" +
+      "  </types>\n" +
+      "</schema>";
+
+
+}