diff --git a/src/image.c b/src/image.c
index f8b3189..d286adb 100644
--- a/src/image.c
+++ b/src/image.c
@@ -183,7 +183,7 @@ PHP_METHOD(Image, convert)
 		size_t len = wkhtmltoimage_get_output(w->converter, &buff);
 
 		if (len) {
-			RETURN_STRINGL(buff, len);
+			RETURN_STRINGL((const char *)buff, len);
 		}
 	}
 } /* }}} */
diff --git a/src/pdf.c b/src/pdf.c
index 3d72bae..53139e2 100644
--- a/src/pdf.c
+++ b/src/pdf.c
@@ -272,7 +272,7 @@ PHP_METHOD(PDF, convert)
 		size_t len = wkhtmltopdf_get_output(w->converter, &buff);
 
 		if (len) {
-			RETURN_STRINGL(buff, len);
+			RETURN_STRINGL((const char *)buff, len);
 		}
 	}
 } /* }}} */