diff --git a/Source/cmIfCommand.cxx b/Source/cmIfCommand.cxx
index b86a7542b8..37b3045513 100644
--- a/Source/cmIfCommand.cxx
+++ b/Source/cmIfCommand.cxx
@@ -170,7 +170,7 @@ bool cmIfCommand::InitialPass(std::vector<std::string> const& args)
     cmRegularExpression regEntry(args[2].c_str());
     
     // check for black line or comment
-    if (!regEntry.find(def))
+    if (def && !regEntry.find(def))
       {
       f = new cmIfFunctionBlocker();
       }