diff --git a/util/PatternMatch.cc b/util/PatternMatch.cc
index c8cb1ce0..b918a9e6 100644
--- a/util/PatternMatch.cc
+++ b/util/PatternMatch.cc
@@ -114,7 +114,7 @@ RegexpCompileError::RegexpCompileError(const char *pattern)  :
 {
   error_ = "TCL failed to compile regular expression '";
   error_ += pattern;
-  error_ =+ "'.";
+  error_ += "'.";
 }
 
 const char *