--- a/xpcom/base/SystemMemoryReporter.cpp
+++ b/xpcom/base/SystemMemoryReporter.cpp
@@ -713,17 +713,17 @@ private:
 
     // The first three fields of each entry interest us:
     //   1) client - Essentially the process name. We limit client names to 63
     //               characters, in theory they should never be greater than 15
     //               due to thread name length limitations.
     //   2) pid    - The ID of the allocating process, read as a uint32_t.
     //   3) size   - The size of the allocation in bytes, read as as a uint64_t.
     const char* const kFormatString = "%63s %" SCNu32 " %" SCNu64;
-    const size_t kNumFields = 3;
+    const int kNumFields = 3;
     const size_t kStringSize = 64;
     const char* const kIonIommuPath = "/sys/kernel/debug/ion/iommu";
 
     FILE* iommu = fopen(kIonIommuPath, "r");
     if (!iommu) {
       if (NS_WARN_IF(errno != ENOENT)) {
         return NS_ERROR_FAILURE;
       }