diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index f1edfe5536df8..90792c3e4bb96 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -3348,7 +3348,8 @@ try_next_encoding:;
 	}
 
 	for (size_t i = 0; i < length; i++) {
-		array[i].demerits = (uint64_t) (array[i].demerits * array[i].multiplier);
+		double demerits = array[i].demerits * (double) array[i].multiplier;
+		array[i].demerits = demerits < (double) UINT64_MAX ? (uint64_t) demerits : UINT64_MAX;
 	}
 
 	return length;
diff --git a/ext/mbstring/tests/gh17503.phpt b/ext/mbstring/tests/gh17503.phpt
new file mode 100644
index 0000000000000..92a2cf39cb10f
--- /dev/null
+++ b/ext/mbstring/tests/gh17503.phpt
@@ -0,0 +1,11 @@
+--TEST--
+GH-17503 (Undefined float conversion in mb_convert_variables)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+$a = array_fill(0, 500, "<blah>");
+var_dump(mb_convert_variables("ASCII", ["UTF-8", "UTF-16"], $a));
+?>
+--EXPECT--
+string(5) "UTF-8"