diff --git a/Source/cmAddCustomCommandCommand.cxx b/Source/cmAddCustomCommandCommand.cxx
index e471dc339..fd1954b1d 100644
--- a/Source/cmAddCustomCommandCommand.cxx
+++ b/Source/cmAddCustomCommandCommand.cxx
@@ -134,7 +134,7 @@ bool cmAddCustomCommandCommand::InitialPass(std::vector<std::string> const& args
           comment = copy;
           break;
         default:
-          this->SetError("Wrong syntax. Unknow type of argument.");
+          this->SetError("Wrong syntax. Unknown type of argument.");
           return false;
         }
       }