diff --git a/scripts/scm-prompt.sh b/scripts/scm-prompt.sh
--- a/scripts/scm-prompt.sh
+++ b/scripts/scm-prompt.sh
@@ -193,7 +193,7 @@
   # - provide a space for the user so that they don't have to have
   #   random extra spaces in their prompt when not in a repo
   # - provide parens so it's differentiated from other crap in their prompt
-  fmt="${1:-' (%s)'}"
+  fmt="${1:- (%s)}"
 
   # find out if we're in a git or hg repo by looking for the control dir
   dir="$PWD"
diff --git a/tests/test-scm-prompt-compat.t b/tests/test-scm-prompt-compat.t
--- a/tests/test-scm-prompt-compat.t
+++ b/tests/test-scm-prompt-compat.t
@@ -35,3 +35,7 @@
   $ hg init repo
   $ oldcmd cd repo
   g g empty g g
+
+Test main prompt with no format string
+  $ _scm_prompt
+   (empty) (no-eol)