diff --git a/litex/build/sim/core/modules/clocker/clocker.c b/litex/build/sim/core/modules/clocker/clocker.c
index 785d0d719..92115a413 100644
--- a/litex/build/sim/core/modules/clocker/clocker.c
+++ b/litex/build/sim/core/modules/clocker/clocker.c
@@ -66,8 +66,8 @@ static int clocker_parse_args(struct session_s *s, const char *args)
     goto out;
   }
 
-  s->freq_hz = json_object_get_uint64(freq_json);
-  s->phase_deg = json_object_get_uint64(phase_json);
+  s->freq_hz = json_object_get_int64(freq_json);
+  s->phase_deg = json_object_get_int64(phase_json);
 
   if (s->freq_hz == 0) {
     ret = RC_JSERROR;
diff --git a/litex/build/sim/core/parse.c b/litex/build/sim/core/parse.c
index 5b45cb778..e0ef15a18 100644
--- a/litex/build/sim/core/parse.c
+++ b/litex/build/sim/core/parse.c
@@ -345,7 +345,7 @@ static int json_get_timebase(json_object *obj, uint64_t *timebase)
       goto out;
     }
 
-    _timebase = json_object_get_uint64(json_timebase);
+    _timebase = json_object_get_int64(json_timebase);
     if (_timebase == 0)
     {
       ret=RC_JSERROR;