diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 269a60ea44..d7e410d948 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -159,7 +159,7 @@ check_unchanged () {
 }
 
 valid_tool () {
-	setup_tool "$1" && return 0
+	setup_tool "$1" 2>/dev/null && return 0
 	cmd=$(get_merge_tool_cmd "$1")
 	test -n "$cmd"
 }
@@ -250,7 +250,12 @@ setup_tool () {
 		. "$MERGE_TOOLS_DIR/${tool%[0-9]}"
 	else
 		setup_user_tool
-		return $?
+		rc=$?
+		if test $rc -ne 0
+		then
+			echo >&2 "error: ${TOOL_MODE}tool.$tool.cmd not set for tool '$tool'"
+		fi
+		return $rc
 	fi
 
 	# Now let the user override the default command for the tool.  If
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 5c5e79e990..4ec11f997e 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -899,4 +899,12 @@ test_expect_success 'mergetool with guiDefault' '
 	git commit -m "branch1 resolved with mergetool"
 '
 
+test_expect_success 'mergetool with non-existent tool' '
+	test_when_finished "git reset --hard" &&
+	git checkout -b test$test_count branch1 &&
+	test_must_fail git merge main &&
+	yes "" | test_must_fail git mergetool --tool=absent >out 2>&1 &&
+	test_grep "mergetool.absent.cmd not set for tool" out
+'
+
 test_done