diff --git a/pdf2image/pdf2image.py b/pdf2image/pdf2image.py
index ae393fd..21d19c8 100644
--- a/pdf2image/pdf2image.py
+++ b/pdf2image/pdf2image.py
@@ -156,6 +156,7 @@ def convert_from_path(
     ):
         if single_file:
             output_file = iter([output_file])
+            thread_count = 1
         else:
             output_file = counter_generator(output_file)