diff --git a/src/main/java/pythagoras/d/MathUtil.java b/src/main/java/pythagoras/d/MathUtil.java
index fcf7413..78d29ee 100644
--- a/src/main/java/pythagoras/d/MathUtil.java
+++ b/src/main/java/pythagoras/d/MathUtil.java
@@ -196,6 +196,8 @@ public class MathUtil
      * The value is also always preceded by a sign (e.g. +1.0 or -0.5).
      */
     public static String toString (double value, int decimalPlaces) {
+        if (Double.isNaN(value)) return "NaN";
+
         StringBuilder buf = new StringBuilder();
         if (value >= 0) buf.append("+");
         else {
diff --git a/src/main/java/pythagoras/f/MathUtil.java b/src/main/java/pythagoras/f/MathUtil.java
index e15e3c2..7723fdc 100644
--- a/src/main/java/pythagoras/f/MathUtil.java
+++ b/src/main/java/pythagoras/f/MathUtil.java
@@ -196,6 +196,8 @@ public class MathUtil
      * The value is also always preceded by a sign (e.g. +1.0 or -0.5).
      */
     public static String toString (float value, int decimalPlaces) {
+        if (Float.isNaN(value)) return "NaN";
+
         StringBuilder buf = new StringBuilder();
         if (value >= 0) buf.append("+");
         else {