diff --git a/dlls/crypt32/msg.c b/dlls/crypt32/msg.c
index 80b2de78e95..fd43a7baeb3 100644
--- a/dlls/crypt32/msg.c
+++ b/dlls/crypt32/msg.c
@@ -2727,12 +2727,10 @@ static BOOL CRYPT_CopySignerInfo(void *pvData, DWORD *pcbData,
     size += CRYPT_SizeOfAttributes(&in->UnauthAttrs);
     if (!pvData)
     {
-        *pcbData = size;
         ret = TRUE;
     }
     else if (*pcbData < size)
     {
-        *pcbData = size;
         SetLastError(ERROR_MORE_DATA);
         ret = FALSE;
     }
@@ -2765,6 +2763,7 @@ static BOOL CRYPT_CopySignerInfo(void *pvData, DWORD *pcbData,
             CRYPT_CopyAttributes(&out->UnauthAttrs, &in->UnauthAttrs, &nextData);
         }
     }
+    *pcbData = size;
     TRACE("returning %d\n", ret);
     return ret;
 }
diff --git a/dlls/crypt32/tests/msg.c b/dlls/crypt32/tests/msg.c
index e9887b06f68..c2405335b91 100644
--- a/dlls/crypt32/tests/msg.c
+++ b/dlls/crypt32/tests/msg.c
@@ -2857,7 +2857,7 @@ static void test_decode_msg_get_param(void)
     HCRYPTPROV hCryptProv;
     HCRYPTKEY key = 0;
     BOOL ret;
-    DWORD size = 0, value;
+    DWORD size = 0, value, req_size;
     LPBYTE buf;
     CMSG_CTRL_DECRYPT_PARA decryptPara = { sizeof(decryptPara), 0 };
 
@@ -2947,7 +2947,10 @@ static void test_decode_msg_get_param(void)
         signer.SerialNumber.cbData = sizeof(serialNum);
         signer.SerialNumber.pbData = serialNum;
         signer.HashAlgorithm.pszObjId = oid_rsa_md5;
+        req_size = size;
+        size += 10;
         CryptMsgGetParam(msg, CMSG_SIGNER_INFO_PARAM, 0, buf, &size);
+        ok(size == req_size, "size = %u, expected %u\n", size, req_size);
         compare_signer_info((CMSG_SIGNER_INFO *)buf, &signer);
         CryptMemFree(buf);
     }