diff --git a/python/src/deltachat/_build.py b/python/src/deltachat/_build.py
index 2a38fbac..47ac424c 100644
--- a/python/src/deltachat/_build.py
+++ b/python/src/deltachat/_build.py
@@ -16,6 +16,7 @@ def ffibuilder():
     )
     builder.cdef("""
         typedef int... time_t;
+        void free(void *ptr);
     """)
     cc = distutils.ccompiler.new_compiler(force=True)
     distutils.sysconfig.customize_compiler(cc)