diff --git a/runtime/utils.cc b/runtime/utils.cc
index 2339903..950e3f9 100644
--- a/runtime/utils.cc
+++ b/runtime/utils.cc
@@ -438,7 +438,7 @@
   return result;
 }
 
-std::string PrettySize(size_t byte_count) {
+std::string PrettySize(int64_t byte_count) {
   // The byte thresholds at which we display amounts.  A byte count is displayed
   // in unit U when kUnitThresholds[U] <= bytes < kUnitThresholds[U+1].
   static const size_t kUnitThresholds[] = {
@@ -447,17 +447,20 @@
     2*1024*1024,    // MB up to...
     1024*1024*1024  // GB from here.
   };
-  static const size_t kBytesPerUnit[] = { 1, KB, MB, GB };
+  static const int64_t kBytesPerUnit[] = { 1, KB, MB, GB };
   static const char* const kUnitStrings[] = { "B", "KB", "MB", "GB" };
-
+  const char* negative_str = "";
+  if (byte_count < 0) {
+    negative_str = "-";
+    byte_count = -byte_count;
+  }
   int i = arraysize(kUnitThresholds);
   while (--i > 0) {
     if (byte_count >= kUnitThresholds[i]) {
       break;
     }
   }
-
-  return StringPrintf("%zd%s", byte_count / kBytesPerUnit[i], kUnitStrings[i]);
+  return StringPrintf("%s%lld%s", negative_str, byte_count / kBytesPerUnit[i], kUnitStrings[i]);
 }
 
 std::string PrettyDuration(uint64_t nano_duration) {