diff --git a/src/core/model/nstime.h b/src/core/model/nstime.h
index 4a5a672ba..65acd55f2 100644
--- a/src/core/model/nstime.h
+++ b/src/core/model/nstime.h
@@ -414,7 +414,7 @@ private:
   {
     bool toMul;
     bool fromMul;
-    uint64_t factor;
+    int64_t factor;
     int64x64_t timeTo;
     int64x64_t timeFrom;
   };
diff --git a/src/core/model/time.cc b/src/core/model/time.cc
index 46cc78d77..9a2420552 100644
--- a/src/core/model/time.cc
+++ b/src/core/model/time.cc
@@ -108,7 +108,7 @@ Time::SetResolution (enum Unit unit, struct Resolution *resolution)
   for (int i = 0; i < Time::LAST; i++)
     {
       int shift = power[i] - power[(int)unit];
-      uint64_t factor = (uint64_t) std::pow (10, std::fabs (shift));
+      int64_t factor = (int64_t) std::pow (10, std::fabs (shift));
       struct Information *info = &resolution->info[i];
       info->factor = factor;
       if (shift == 0)