diff --git a/build/org.eclipse.cdt.make.ui/src/org/eclipse/cdt/make/internal/ui/text/makefile/MacroDefinitionRule.java b/build/org.eclipse.cdt.make.ui/src/org/eclipse/cdt/make/internal/ui/text/makefile/MacroDefinitionRule.java
index 0eb413832cf..b4fd6ea6a04 100644
--- a/build/org.eclipse.cdt.make.ui/src/org/eclipse/cdt/make/internal/ui/text/makefile/MacroDefinitionRule.java
+++ b/build/org.eclipse.cdt.make.ui/src/org/eclipse/cdt/make/internal/ui/text/makefile/MacroDefinitionRule.java
@@ -157,9 +157,13 @@ class MacroDefinitionRule implements IPredicateRule {
 
 		return true;
 	}
+
 	protected boolean isValidCharacter(int c) {
-		char c0 = (char) c;
-		return Character.isLetterOrDigit(c0) || (c0 == '_');
+		// From GNUMakefile manual:
+		// A variable name may be any sequence of characters not containing ‘:’, ‘#’, ‘=’, or leading or trailing whitespace.
+		// However, variable names containing characters other than letters, numbers, and underscores should be avoided,
+		// as they may be given special meanings in the future, and with some shells they cannot be passed through the environment to a sub-make
+		return !Character.isWhitespace(c) && c != ':' && c != '#' && c != '=';
 	}
 
 }