diff --git a/lib/zip_add_entry.c b/lib/zip_add_entry.c
index bf12dd541..24e564867 100644
--- a/lib/zip_add_entry.c
+++ b/lib/zip_add_entry.c
@@ -64,7 +64,7 @@ _zip_add_entry(zip_t *za) {
             return -1;
         }
         rentries = (zip_entry_t *)realloc(za->entry, sizeof(struct zip_entry) * (size_t)nalloc);
-        if (!rentries) {
+        if (rentries == NULL) {
             zip_error_set(&za->error, ZIP_ER_MEMORY, 0);
             return -1;
         }
diff --git a/lib/zip_dirent.c b/lib/zip_dirent.c
index f5f23cb3a..41d4e7e96 100644
--- a/lib/zip_dirent.c
+++ b/lib/zip_dirent.c
@@ -51,8 +51,9 @@ void
 _zip_cdir_free(zip_cdir_t *cd) {
     zip_uint64_t i;
 
-    if (!cd)
+    if (cd == NULL) {
         return;
+    }
 
     for (i = 0; i < cd->nentry; i++)
         _zip_entry_finalize(cd->entry + i);
@@ -139,6 +140,7 @@ _zip_cdir_write(zip_t *za, const zip_filelist_t *filelist, zip_uint64_t survivor
         zip_entry_t *entry = za->entry + filelist[i].idx;
 
         if (_zip_dirent_write(za, entry->changes ? entry->changes : entry->orig, ZIP_FL_CENTRAL) < 0) {
+            za->write_crc = NULL;
             return -1;
         }
     }
@@ -446,7 +448,7 @@ _zip_dirent_read(zip_dirent_t *zde, zip_source_t *src, zip_buffer_t *buffer, boo
 
     if (filename_len) {
         zde->filename = _zip_read_string(buffer, src, filename_len, 1, error);
-        if (!zde->filename) {
+        if (zde->filename == NULL) {
             if (zip_error_code_zip(error) == ZIP_ER_EOF) {
                 zip_error_set(error, ZIP_ER_INCONS, ZIP_ER_DETAIL_VARIABLE_SIZE_OVERFLOW);
             }
@@ -490,7 +492,7 @@ _zip_dirent_read(zip_dirent_t *zde, zip_source_t *src, zip_buffer_t *buffer, boo
 
     if (comment_len) {
         zde->comment = _zip_read_string(buffer, src, comment_len, 0, error);
-        if (!zde->comment) {
+        if (zde->comment == NULL) {
             if (!from_buffer) {
                 _zip_buffer_free(buffer);
             }
diff --git a/lib/zip_fread.c b/lib/zip_fread.c
index 5b7da46ae..53c8002a7 100644
--- a/lib/zip_fread.c
+++ b/lib/zip_fread.c
@@ -39,11 +39,13 @@ ZIP_EXTERN zip_int64_t
 zip_fread(zip_file_t *zf, void *outbuf, zip_uint64_t toread) {
     zip_int64_t n;
 
-    if (!zf)
+    if (zf == NULL) {
         return -1;
+    }
 
-    if (zf->error.zip_err != 0)
+    if (zf->error.zip_err != 0) {
         return -1;
+    }
 
     if (toread > ZIP_INT64_MAX) {
         zip_error_set(&zf->error, ZIP_ER_INVAL, 0);
diff --git a/lib/zip_fseek.c b/lib/zip_fseek.c
index e68ffd367..c21bbab15 100644
--- a/lib/zip_fseek.c
+++ b/lib/zip_fseek.c
@@ -36,11 +36,13 @@
 
 ZIP_EXTERN zip_int8_t
 zip_fseek(zip_file_t *zf, zip_int64_t offset, int whence) {
-    if (!zf)
+    if (zf == NULL) {
         return -1;
+    }
 
-    if (zf->error.zip_err != 0)
+    if (zf->error.zip_err != 0) {
         return -1;
+    }
 
     if (zip_source_seek(zf->src, offset, whence) < 0) {
         zip_error_set_from_source(&zf->error, zf->src);
@@ -53,7 +55,7 @@ zip_fseek(zip_file_t *zf, zip_int64_t offset, int whence) {
 
 ZIP_EXTERN int
 zip_file_is_seekable(zip_file_t *zfile) {
-    if (!zfile) {
+    if (zfile == NULL) {
         return -1;
     }
     
diff --git a/lib/zip_ftell.c b/lib/zip_ftell.c
index bf3b03d34..f5109ad5a 100644
--- a/lib/zip_ftell.c
+++ b/lib/zip_ftell.c
@@ -38,11 +38,13 @@ ZIP_EXTERN zip_int64_t
 zip_ftell(zip_file_t *zf) {
     zip_int64_t res;
 
-    if (!zf)
+    if (zf == NULL) {
         return -1;
+    }
 
-    if (zf->error.zip_err != 0)
+    if (zf->error.zip_err != 0) {
         return -1;
+    }
 
     res = zip_source_tell(zf->src);
     if (res < 0) {
diff --git a/lib/zip_io_util.c b/lib/zip_io_util.c
index 9fcd10b4b..ce8beb518 100644
--- a/lib/zip_io_util.c
+++ b/lib/zip_io_util.c
@@ -70,7 +70,7 @@ _zip_read_data(zip_buffer_t *buffer, zip_source_t *src, size_t length, bool nulp
     }
 
     r = (zip_uint8_t *)malloc(length + (nulp ? 1 : 0));
-    if (!r) {
+    if (r == NULL) {
         zip_error_set(error, ZIP_ER_MEMORY, 0);
         return NULL;
     }
diff --git a/lib/zip_memdup.c b/lib/zip_memdup.c
index 75d72c616..cec701354 100644
--- a/lib/zip_memdup.c
+++ b/lib/zip_memdup.c
@@ -45,7 +45,7 @@ _zip_memdup(const void *mem, size_t len, zip_error_t *error) {
         return NULL;
 
     ret = malloc(len);
-    if (!ret) {
+    if (ret == NULL) {
         zip_error_set(error, ZIP_ER_MEMORY, 0);
         return NULL;
     }
diff --git a/lib/zip_new.c b/lib/zip_new.c
index 4f69c8a2f..1371698f5 100644
--- a/lib/zip_new.c
+++ b/lib/zip_new.c
@@ -46,7 +46,7 @@ _zip_new(zip_error_t *error) {
     zip_t *za;
 
     za = (zip_t *)malloc(sizeof(struct zip));
-    if (!za) {
+    if (za == NULL) {
         zip_error_set(error, ZIP_ER_MEMORY, 0);
         return NULL;
     }
diff --git a/lib/zip_source_file.h b/lib/zip_source_file.h
index cca9fd2b5..c9ce57c25 100644
--- a/lib/zip_source_file.h
+++ b/lib/zip_source_file.h
@@ -1,3 +1,6 @@
+#ifndef _HAD_ZIP_SOURCE_FILE_H
+#define _HAD_ZIP_SOURCE_FILE_H
+
 /*
   zip_source_file.h -- header for common file operations
   Copyright (C) 2020-2022 Dieter Baron and Thomas Klausner
@@ -88,3 +91,5 @@ struct zip_source_file_operations {
 };
 
 zip_source_t *zip_source_file_common_new(const char *fname, void *file, zip_uint64_t start, zip_int64_t len, const zip_stat_t *st, zip_source_file_operations_t *ops, void *ops_userdata, zip_error_t *error);
+
+#endif /* _HAD_ZIP_SOURCE_FILE_H */
diff --git a/lib/zipint.h b/lib/zipint.h
index d7724a3d8..0c4b3fea0 100644
--- a/lib/zipint.h
+++ b/lib/zipint.h
@@ -673,4 +673,4 @@ int _zip_unchange(zip_t *, zip_uint64_t, int);
 void _zip_unchange_data(zip_entry_t *);
 int _zip_write(zip_t *za, const void *data, zip_uint64_t length);
 
-#endif /* zipint.h */
+#endif /* _HAD_ZIPINT_H */