diff --git a/src/com/bytezone/diskbrowser/visicalc/Cell.java b/src/com/bytezone/diskbrowser/visicalc/Cell.java
index a41ffe9..d6cbfc6 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Cell.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Cell.java
@@ -121,12 +121,10 @@ class Cell implements Comparable<Cell>, Value
         return justify (repeat, colWidth);
 
       case VALUE:
-        if (value.isError () || value.isNotAvailable ())
+        if (value.isError () || value.isNotAvailable () || value.isNotANumber ())
           return justify (value.getText (), colWidth);
 
         Double thisValue = value.getValue ();
-        if (thisValue.isNaN ())
-          return justify ("NaN", colWidth);
 
         char format = cellFormat != ' ' ? cellFormat : defaultFormat;
         if (format == 'I')
@@ -210,6 +208,15 @@ class Cell implements Comparable<Cell>, Value
     return value.isNotAvailable ();
   }
 
+  @Override
+  public boolean isNotANumber ()
+  {
+    //    assert type == CellType.VALUE : "Cell type: " + type;
+    //    if (!isValue ())
+    //      return true;
+    return value.isNotANumber ();
+  }
+
   @Override
   public Value calculate ()
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Expression.java b/src/com/bytezone/diskbrowser/visicalc/Expression.java
index 18b45da..9fe21cb 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Expression.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Expression.java
@@ -173,7 +173,11 @@ class Expression implements Value
         else if (operator.equals ("^"))
           value = Math.pow (value, nextValue);
       }
-      valueType = ValueType.VALUE;
+
+      if (Double.isNaN (value))
+        valueType = ValueType.NAN;
+      else
+        valueType = ValueType.VALUE;
     }
     catch (Exception e)
     {
@@ -202,6 +206,12 @@ class Expression implements Value
     return valueType == ValueType.NA;
   }
 
+  @Override
+  public boolean isNotANumber ()
+  {
+    return valueType == ValueType.NAN;
+  }
+
   @Override
   public boolean isError ()
   {
@@ -211,14 +221,15 @@ class Expression implements Value
   @Override
   public double getValue ()
   {
-    assert valueType == ValueType.VALUE : "Expression ValueType = " + valueType;
+    //    assert valueType == ValueType.VALUE : "Expression ValueType = " + valueType;
     return value;
   }
 
   @Override
   public String getText ()
   {
-    return isNotAvailable () ? "NA" : isError () ? "Error" : "";
+    return isNotAvailable () ? "NA" : isError () ? "Error" : isNotANumber () ? "NaN" : "";
+    //    return isNotAvailable () ? "NA" : isError () ? "Error" : "";
   }
 
   private String checkBrackets (String input)
diff --git a/src/com/bytezone/diskbrowser/visicalc/Function.java b/src/com/bytezone/diskbrowser/visicalc/Function.java
index ad94988..762c3b0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Function.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Function.java
@@ -139,6 +139,12 @@ abstract class Function implements Value
     return valueType == ValueType.NA;
   }
 
+  @Override
+  public boolean isNotANumber ()
+  {
+    return valueType == ValueType.NAN;
+  }
+
   @Override
   public double getValue ()
   {
@@ -150,7 +156,7 @@ abstract class Function implements Value
   @Override
   public String getText ()
   {
-    return isNotAvailable () ? "" : isError () ? "Error" : "";
+    return isNotAvailable () ? "NA" : isError () ? "Error" : isNotANumber () ? "NaN" : "";
   }
 
   protected Range getRange (String text)
diff --git a/src/com/bytezone/diskbrowser/visicalc/Number.java b/src/com/bytezone/diskbrowser/visicalc/Number.java
index b0fcaac..aa4ac49 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Number.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Number.java
@@ -36,6 +36,12 @@ class Number implements Value
     return valueType == ValueType.NA;
   }
 
+  @Override
+  public boolean isNotANumber ()
+  {
+    return valueType == ValueType.NAN;
+  }
+
   @Override
   public String toString ()
   {
@@ -51,7 +57,8 @@ class Number implements Value
   @Override
   public String getText ()
   {
-    return valueType == ValueType.ERROR ? "Error" : "";
+    return isNotAvailable () ? "NA" : isError () ? "Error" : isNotANumber () ? "NaN" : "";
+    //    return valueType == ValueType.ERROR ? "Error" : "";
   }
 
   @Override
diff --git a/src/com/bytezone/diskbrowser/visicalc/Value.java b/src/com/bytezone/diskbrowser/visicalc/Value.java
index fccac5e..50fac61 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Value.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Value.java
@@ -4,7 +4,7 @@ interface Value
 {
   enum ValueType
   {
-    VALUE, ERROR, NA
+    VALUE, ERROR, NA, NAN
   }
 
   public ValueType getValueType ();
@@ -19,5 +19,7 @@ interface Value
 
   public boolean isNotAvailable ();
 
+  public boolean isNotANumber ();
+
   public Value calculate ();
 }
\ No newline at end of file