diff --git a/rbtools/clients/git.py b/rbtools/clients/git.py
index 36d6296b6cbbc99c592bea3dfac5fbb8c5c6c50d..51763f384dea1d4533a76e532cdf90d34707cd24 100644
--- a/rbtools/clients/git.py
+++ b/rbtools/clients/git.py
@@ -473,7 +473,7 @@ class GitClient(SCMClient):
         # things from breaking horribly if someone configures a graphical diff
         # viewer like p4merge or kaleidoscope. This can be overridden by
         # setting GIT_USE_EXT_DIFF = True in ~/.reviewboardrc
-        if self.user_config.get('GIT_USE_EXT_DIFF', False):
+        if not self.user_config.get('GIT_USE_EXT_DIFF', False):
             diff_cmd.append('--no-ext-diff')
 
         diff_lines = execute(diff_cmd + files,