diff --git a/llm/server.go b/llm/server.go
index 832863720..bb9062adc 100644
--- a/llm/server.go
+++ b/llm/server.go
@@ -700,20 +700,24 @@ func (s *llmServer) Completion(ctx context.Context, req CompletionRequest, fn fu
 	}
 
 	if len(req.Format) > 0 {
-		switch {
-		case bytes.Equal(req.Format, []byte(`""`)):
-			// fallthrough
-		case bytes.Equal(req.Format, []byte(`"json"`)):
+		switch string(req.Format) {
+		case `null`, `""`:
+			// Field was set, but "missing" a value. We accept
+			// these as "not set".
+			break
+		case `"json"`:
 			request["grammar"] = grammarJSON
-		case bytes.HasPrefix(req.Format, []byte("{")):
+		default:
+			if req.Format[0] != '{' {
+				return fmt.Errorf("invalid format: %q; expected \"json\" or a valid JSON Schema object", req.Format)
+			}
+
 			// User provided a JSON schema
 			g := llama.SchemaToGrammar(req.Format)
 			if g == nil {
 				return fmt.Errorf("invalid JSON schema in format")
 			}
 			request["grammar"] = string(g)
-		default:
-			return fmt.Errorf("invalid format: %q; expected \"json\" or a valid JSON Schema", req.Format)
 		}
 	}
 
diff --git a/llm/server_test.go b/llm/server_test.go
index e6f79a585..6c8f7590b 100644
--- a/llm/server_test.go
+++ b/llm/server_test.go
@@ -39,25 +39,34 @@ func TestLLMServerCompletionFormat(t *testing.T) {
 
 	cancel() // prevent further processing if request makes it past the format check
 
-	checkCanceled := func(err error) {
+	checkValid := func(err error) {
 		t.Helper()
 		if !errors.Is(err, context.Canceled) {
 			t.Fatalf("Completion: err = %v; expected context.Canceled", err)
 		}
 	}
 
-	valids := []string{`"json"`, `{"type":"object"}`, ``, `""`}
+	valids := []string{
+		// "missing"
+		``,
+		`""`,
+		`null`,
+
+		// JSON
+		`"json"`,
+		`{"type":"object"}`,
+	}
 	for _, valid := range valids {
 		err := s.Completion(ctx, CompletionRequest{
 			Options: new(api.Options),
 			Format:  []byte(valid),
 		}, nil)
-		checkCanceled(err)
+		checkValid(err)
 	}
 
 	err := s.Completion(ctx, CompletionRequest{
 		Options: new(api.Options),
 		Format:  nil, // missing format
 	}, nil)
-	checkCanceled(err)
+	checkValid(err)
 }