diff --git a/src/main/java/org/apache/commons/math3/linear/DiagonalMatrix.java b/src/main/java/org/apache/commons/math3/linear/DiagonalMatrix.java
index 237fa2665..b8dccbb2f 100644
--- a/src/main/java/org/apache/commons/math3/linear/DiagonalMatrix.java
+++ b/src/main/java/org/apache/commons/math3/linear/DiagonalMatrix.java
@@ -103,7 +103,7 @@ public class DiagonalMatrix extends AbstractRealMatrix
 
     /** {@inheritDoc} */
     @Override
-    public DiagonalMatrix copy() {
+    public RealMatrix copy() {
         return new DiagonalMatrix(data);
     }
 
diff --git a/src/test/java/org/apache/commons/math3/linear/DiagonalMatrixTest.java b/src/test/java/org/apache/commons/math3/linear/DiagonalMatrixTest.java
index 097b5d3f8..3379dbf5e 100644
--- a/src/test/java/org/apache/commons/math3/linear/DiagonalMatrixTest.java
+++ b/src/test/java/org/apache/commons/math3/linear/DiagonalMatrixTest.java
@@ -97,7 +97,7 @@ public class DiagonalMatrixTest {
     public void testCopy() {
         final double[] d = { -1.2, 3.4, 5 };
         final DiagonalMatrix m = new DiagonalMatrix(d, false);
-        final DiagonalMatrix p = m.copy();
+        final DiagonalMatrix p = (DiagonalMatrix) m.copy();
         for (int i = 0; i < m.getRowDimension(); ++i) {
             Assert.assertEquals(m.getEntry(i, i), p.getEntry(i, i), 1.0e-20);
         }