diff --git a/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/dfp/Dfp.java b/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/dfp/Dfp.java
index 41f15c911..02f7df81e 100644
--- a/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/dfp/Dfp.java
+++ b/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/dfp/Dfp.java
@@ -288,7 +288,7 @@ public class Dfp implements RealFieldElement<Dfp> {
 
         if (exponent == 1024) {
             // infinity or NAN
-            if (x != x) {
+            if (Double.isNaN(x)) {
                 sign = (byte) 1;
                 nans = QNAN;
             } else if (x < 0) {
diff --git a/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/jdkmath/AccurateMathCalc.java b/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/jdkmath/AccurateMathCalc.java
index 69cc11b72..e9ec154ff 100644
--- a/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/jdkmath/AccurateMathCalc.java
+++ b/commons-math-legacy-core/src/main/java/org/apache/commons/math4/legacy/core/jdkmath/AccurateMathCalc.java
@@ -636,7 +636,7 @@ final class AccurateMathCalc {
      * @return formatted number
      */
     static String format(double d) {
-        if (d != d) {
+        if (Double.isNaN(d)) {
             return "Double.NaN,";
         } else {
             return ((d >= 0) ? "+" : "") + Double.toString(d) + "d,";