diff --git a/git-merge.sh b/git-merge.sh
index 1c123a37e6..03cd39873a 100755
--- a/git-merge.sh
+++ b/git-merge.sh
@@ -37,6 +37,7 @@ use_strategies=
 
 allow_fast_forward=t
 allow_trivial_merge=t
+squash= no_commit=
 
 dropsave() {
 	rm -f -- "$GIT_DIR/MERGE_HEAD" "$GIT_DIR/MERGE_MSG" \
@@ -152,17 +153,21 @@ parse_config () {
 		--summary)
 			show_diffstat=t ;;
 		--squash)
-			allow_fast_forward=t squash=t no_commit=t ;;
+			test "$allow_fast_forward" = t ||
+				die "You cannot combine --squash with --no-ff."
+			squash=t no_commit=t ;;
 		--no-squash)
-			allow_fast_forward=t squash= no_commit= ;;
+			squash= no_commit= ;;
 		--commit)
-			allow_fast_forward=t squash= no_commit= ;;
+			no_commit= ;;
 		--no-commit)
-			allow_fast_forward=t squash= no_commit=t ;;
+			no_commit=t ;;
 		--ff)
-			allow_fast_forward=t squash= no_commit= ;;
+			allow_fast_forward=t ;;
 		--no-ff)
-			allow_fast_forward=false squash= no_commit= ;;
+			test "$squash" != t ||
+				die "You cannot combine --squash with --no-ff."
+			allow_fast_forward=f ;;
 		-s|--strategy)
 			shift
 			case " $all_strategies " in
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 50c51c82fa..5d166280cb 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -419,6 +419,7 @@ test_debug 'gitk --all'
 
 test_expect_success 'merge c0 with c1 (no-ff)' '
 	git reset --hard c0 &&
+	git config branch.master.mergeoptions "" &&
 	test_tick &&
 	git merge --no-ff c1 &&
 	verify_merge file result.1 &&
@@ -427,6 +428,11 @@ test_expect_success 'merge c0 with c1 (no-ff)' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'combining --squash and --no-ff is refused' '
+	test_must_fail git merge --squash --no-ff c1 &&
+	test_must_fail git merge --no-ff --squash c1
+'
+
 test_expect_success 'merge c0 with c1 (ff overrides no-ff)' '
 	git reset --hard c0 &&
 	git config branch.master.mergeoptions "--no-ff" &&