diff --git a/src/internal/perk_output_impl.h b/src/internal/perk_output_impl.h
index 7540015..d6f328a 100644
--- a/src/internal/perk_output_impl.h
+++ b/src/internal/perk_output_impl.h
@@ -1,3 +1,4 @@
+#include <unistd.h>
 #include <perk/perk.h>
 
 static inline FILE * pe_output_prolog(