diff --git a/js/src/jit/Safepoints.cpp b/js/src/jit/Safepoints.cpp
index 0d9a16f701d1..89b53ba1e309 100644
--- a/js/src/jit/Safepoints.cpp
+++ b/js/src/jit/Safepoints.cpp
@@ -43,12 +43,20 @@ SafepointWriter::writeOsiCallPointOffset(uint32_t osiCallPointOffset)
 static void
 WriteRegisterMask(CompactBufferWriter &stream, uint32_t bits)
 {
-    if (sizeof(PackedRegisterMask) == 8)
+    if (sizeof(PackedRegisterMask) == 1)
         stream.writeByte(bits);
     else
         stream.writeUnsigned(bits);
 }
 
+static int32_t
+ReadRegisterMask(CompactBufferReader &stream)
+{
+    if (sizeof(PackedRegisterMask) == 1)
+        return stream.readByte();
+    return stream.readUnsigned();
+}
+
 void
 SafepointWriter::writeGcRegs(LSafepoint *safepoint)
 {
@@ -342,20 +350,20 @@ SafepointReader::SafepointReader(IonScript *script, const SafepointIndex *si)
     osiCallPointOffset_ = stream_.readUnsigned();
 
     // gcSpills is a subset of allGprSpills.
-    allGprSpills_ = GeneralRegisterSet(stream_.readUnsigned());
+    allGprSpills_ = GeneralRegisterSet(ReadRegisterMask(stream_));
     if (allGprSpills_.empty()) {
         gcSpills_ = allGprSpills_;
         valueSpills_ = allGprSpills_;
         slotsOrElementsSpills_ = allGprSpills_;
     } else {
-        gcSpills_ = GeneralRegisterSet(stream_.readUnsigned());
-        slotsOrElementsSpills_ = GeneralRegisterSet(stream_.readUnsigned());
+        gcSpills_ = GeneralRegisterSet(ReadRegisterMask(stream_));
+        slotsOrElementsSpills_ = GeneralRegisterSet(ReadRegisterMask(stream_));
 #ifdef JS_PUNBOX64
-        valueSpills_ = GeneralRegisterSet(stream_.readUnsigned());
+        valueSpills_ = GeneralRegisterSet(ReadRegisterMask(stream_));
 #endif
     }
 
-    allFloatSpills_ = FloatRegisterSet(stream_.readUnsigned());
+    allFloatSpills_ = FloatRegisterSet(ReadRegisterMask(stream_));
 
     advanceFromGcRegs();
 }