diff --git a/Misc/ACKS b/Misc/ACKS
index 6f7d9d12511..7096d728d28 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -874,6 +874,7 @@ Monty Taylor
 Amy Taylor
 Anatoly Techtonik
 Mikhail Terekhov
+Richard M. Tew
 Tobias Thelen
 James Thomas
 Robin Thomas
diff --git a/Misc/NEWS b/Misc/NEWS
index f6fe5b7014f..23b336bb52b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #12839: Fix crash in zlib module due to version mismatch.
+  Fix by Richard M. Tew.
+
 - Issue #11657: Fix sending file descriptors over 255 over a multiprocessing
   Pipe.
 
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index ba0e59ce064..a1e605b3d2b 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -52,7 +52,13 @@ typedef struct
 static void
 zlib_error(z_stream zst, int err, char *msg)
 {
-    const char *zmsg = zst.msg;
+    const char *zmsg = Z_NULL;
+    /* In case of a version mismatch, zst.msg won't be initialized.
+       Check for this case first, before looking at zst.msg. */
+    if (err == Z_VERSION_ERROR)
+        zmsg = "library version mismatch";
+    if (zmsg == Z_NULL)
+        zmsg = zst.msg;
     if (zmsg == Z_NULL) {
         switch (err) {
         case Z_BUF_ERROR: