diff --git a/extensions/openpower-pels/json_utils.hpp b/extensions/openpower-pels/json_utils.hpp
index f6db297..6415c9e 100644
--- a/extensions/openpower-pels/json_utils.hpp
+++ b/extensions/openpower-pels/json_utils.hpp
@@ -71,10 +71,15 @@
     static_assert(std::is_integral<T>::value, "Integral required.");
 
     int len = asprintf(&value, format, number);
-    if (len)
+    if (len >= 0)
     {
         numString = value;
     }
+    else
+    {
+        throw std::invalid_argument(
+            std::string("getNumberString: invalid format string: ") + format);
+    }
     free(value);
 
     return numString;