diff --git a/src/hb-subset.cc b/src/hb-subset.cc
index adc7c51..2bed358 100644
--- a/src/hb-subset.cc
+++ b/src/hb-subset.cc
@@ -51,6 +51,9 @@
   unsigned int src_glyphs = plan->source->get_num_glyphs ();
   unsigned int dst_glyphs = plan->glyphset->get_population ();
 
+  if (unlikely (!src_glyphs))
+    return 512 + table_len;
+
   return 512 + (unsigned int) (table_len * sqrt ((double) dst_glyphs / src_glyphs));
 }