diff --git a/gemma/model_store.cc b/gemma/model_store.cc
index d8e69e0..0ffdedb 100644
--- a/gemma/model_store.cc
+++ b/gemma/model_store.cc
@@ -24,6 +24,7 @@
 #include <cstdlib>
 #include <cstring>  // strcmp
 #include <string>
+#include <system_error>  // std::errc  // NOLINT
 
 #include "compression/types.h"
 #include "gemma/configs.h"  // ModelConfig, kMaxQKVDim
@@ -174,12 +175,12 @@ class TypePrefix {
 // Returns 0 if the blob does not seem to be a per-layer tensor, otherwise the
 // layer index.
 static size_t LayerIdxFromKey(const std::string& key) {
-  const auto parse_num = [&key](size_t begin, size_t end) -> size_t {
+  const auto parse_num = [&key](size_t begin, size_t end) -> int {
     HWY_DASSERT(begin <= end);
     HWY_DASSERT(end <= key.size());
-    size_t val = 0;
-    (void)std::from_chars(key.data() + begin, key.data() + end, val);
-    return val;
+    int val = 0;
+    auto [ptr, ec] = std::from_chars(key.data() + begin, key.data() + end, val);
+    return (ec == std::errc()) ? val : -1;
   };
 
   const size_t suffix_pos = key.rfind('_');
@@ -187,10 +188,10 @@ static size_t LayerIdxFromKey(const std::string& key) {
   if (suffix_pos == std::string::npos) return 0;
   if (suffix_pos == key.size() - 1) return 0;
 
-  size_t layer_idx = parse_num(suffix_pos + 1, key.size());
+  int layer_idx = parse_num(suffix_pos + 1, key.size());
 
   HWY_ASSERT(layer_idx < 999);
-  return layer_idx;
+  return layer_idx == -1 ? 0 : static_cast<size_t>(layer_idx);
 }
 
 // Returns the number of layers based on the largest blob name suffix seen.