diff --git a/src/main/java/org/apache/commons/lang3/math/NumberUtils.java b/src/main/java/org/apache/commons/lang3/math/NumberUtils.java
index f1f36a888..d9b3c6de5 100644
--- a/src/main/java/org/apache/commons/lang3/math/NumberUtils.java
+++ b/src/main/java/org/apache/commons/lang3/math/NumberUtils.java
@@ -1409,8 +1409,8 @@ public class NumberUtils {
             }
             if (chars[i] == 'l'
                 || chars[i] == 'L') {
-                // not allowing L with an exponent
-                return foundDigit && !hasExp;
+                // not allowing L with an exponent or decimal point
+                return foundDigit && !hasExp && !hasDecPoint;
             }
             // last character is illegal
             return false;
diff --git a/src/test/java/org/apache/commons/lang3/math/NumberUtilsTest.java b/src/test/java/org/apache/commons/lang3/math/NumberUtilsTest.java
index 779eb746b..bb94ab31c 100644
--- a/src/test/java/org/apache/commons/lang3/math/NumberUtilsTest.java
+++ b/src/test/java/org/apache/commons/lang3/math/NumberUtilsTest.java
@@ -1139,6 +1139,10 @@ public class NumberUtilsTest extends TestCase {
         // LANG-521
         val = "2.";
         assertTrue("isNumber(String) LANG-521 failed", NumberUtils.isNumber(val));
+
+        // LANG-664
+        val = "1.1L";
+        assertFalse("isNumber(String) LANG-664 failed", NumberUtils.isNumber(val));
     }
 
     private boolean checkCreateNumber(String val) {