diff --git a/pkgs/by-name/om/om4/include-exit.patch b/pkgs/by-name/om/om4/include-exit.patch
new file mode 100644
index 000000000000..8b5e2f9b4f54
--- /dev/null
+++ b/pkgs/by-name/om/om4/include-exit.patch
@@ -0,0 +1,14 @@
+Index: a/parser.y
+===================================================================
+--- a/parser.y
++++ a/parser.y
+@@ -16,8 +16,9 @@
+  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+  */
+ #include <math.h>
+ #include <stdint.h>
++#include <stdlib.h>
+ #define YYSTYPE	int32_t
+ extern int32_t end_result;
+ extern int yylex(void);
+ extern int yyerror(const char *);
diff --git a/pkgs/by-name/om/om4/package.nix b/pkgs/by-name/om/om4/package.nix
index 078bc8510a4a..1efc9047ec26 100644
--- a/pkgs/by-name/om/om4/package.nix
+++ b/pkgs/by-name/om/om4/package.nix
@@ -13,10 +13,15 @@ stdenv.mkDerivation rec {
   src = fetchFromGitHub {
     owner = "ibara";
     repo = "m4";
-    rev = "${pname}-${version}";
-    sha256 = "04h76hxwb5rs3ylkw1dv8k0kmkzl84ccqlwdwxb6i0x57rrqbgzx";
+    tag = "om4-${version}";
+    hash = "sha256-/b+Fcz6lg2hW541TzBhB9M86wUS7BT6pHzqXxTs0BxI=";
   };
 
+  patches = [
+    # parser.y:51:25: error: implicit declaration of function 'exit' []
+    ./include-exit.patch
+  ];
+
   strictDeps = true;
   nativeBuildInputs = [
     bison
@@ -25,17 +30,17 @@ stdenv.mkDerivation rec {
 
   configureFlags = [ "--enable-m4" ];
 
-  meta = with lib; {
+  meta = {
     description = "Portable OpenBSD m4 macro processor";
     homepage = "https://github.com/ibara/m4";
-    license = with licenses; [
+    license = with lib.licenses; [
       bsd2
       bsd3
       isc
       publicDomain
     ];
     mainProgram = "m4";
-    platforms = platforms.unix;
+    platforms = lib.platforms.unix;
     maintainers = [ ];
   };
 }