--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -10135,18 +10135,18 @@ IonBuilder::lookupTypedObjectField(MDefi
     // Determine the type/offset of the field `name`, if any.
     int32_t offset;
     if (!objDescrs.fieldNamed(*this, NameToId(name), &offset,
                               fieldDescrs, fieldIndex))
         return false;
     if (fieldDescrs->empty())
         return true;
 
-    *fieldOffset = int32_t(offset);
-    JS_ASSERT(*fieldOffset >= 0);
+    JS_ASSERT(offset >= 0);
+    *fieldOffset = offset;
 
     return true;
 }
 
 MDefinition *
 IonBuilder::typeObjectForElementFromArrayStructType(MDefinition *typeObj)
 {
     MInstruction *elemType = MLoadFixedSlot::New(alloc(), typeObj, JS_DESCR_SLOT_ARRAY_ELEM_TYPE);

--- a/js/src/jit/TypeDescrSet.cpp
+++ b/js/src/jit/TypeDescrSet.cpp
@@ -320,17 +320,17 @@ TypeDescrSet::fieldNamed(IonBuilder &bui
                          int32_t *offset,
                          TypeDescrSet *out,
                          size_t *index)
 {
     JS_ASSERT(kind() == TypeDescr::Struct);
 
     // Initialize `*offset` and `*out` for the case where incompatible
     // or absent fields are found.
-    *offset = SIZE_MAX;
+    *offset = -1;
     *index = SIZE_MAX;
     *out = TypeDescrSet();
 
     // Remember offset of the first field.
     int32_t offset0;
     size_t index0;
     TypeDescrSetBuilder fieldTypes;
     {