diff --git a/modules/ksb/BuildSystem/KDE4.pm b/modules/ksb/BuildSystem/KDE4.pm
--- a/modules/ksb/BuildSystem/KDE4.pm
+++ b/modules/ksb/BuildSystem/KDE4.pm
@@ -170,7 +170,7 @@
     # Get the user's CXXFLAGS, use them if specified and not already given
     # on the command line.
     my $cxxflags = $module->getOption('cxxflags');
-    if ($cxxflags and not grep { /^-DCMAKE_CXX_FLAGS(:\w+)?=/ } @commands)
+    if ($cxxflags and not $cxxflags eq ' ' and not grep { /^-DCMAKE_CXX_FLAGS(:\w+)?=/ } @commands)
     {
         push @commands, "-DCMAKE_CXX_FLAGS:STRING=$cxxflags";
     }