diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -208,9 +208,10 @@ def _picktool(repo, ui, path, binary, sy
     if uimerge:
         # external tools defined in uimerge won't be able to handle
         # change/delete conflicts
-        if uimerge not in names and not changedelete:
-            return (uimerge, uimerge)
-        tools.insert(0, (None, uimerge)) # highest priority
+        if check(uimerge, path, symlink, binary, changedelete):
+            if uimerge not in names and not changedelete:
+                return (uimerge, uimerge)
+            tools.insert(0, (None, uimerge)) # highest priority
     tools.append((None, "hgmerge")) # the old default, if found
     for p, t in tools:
         if check(t, None, symlink, binary, changedelete):
diff --git a/tests/test-merge-tools.t b/tests/test-merge-tools.t
--- a/tests/test-merge-tools.t
+++ b/tests/test-merge-tools.t
@@ -1748,7 +1748,9 @@ missingbinary is a merge-tool that doesn
   missingbinary.executable=doesnotexist
   # hg update -C 1
   $ hg merge -y -r 2 --config ui.merge=missingbinary
+  couldn't find merge tool missingbinary (for pattern f)
   merging f
+  couldn't find merge tool missingbinary (for pattern f)
   revision 1
   space
   revision 0