diff --git a/tests/fuzz/crypt2_load_fuzz.cc b/tests/fuzz/crypt2_load_fuzz.cc
index d3de9a96..0ad4f827 100644
--- a/tests/fuzz/crypt2_load_fuzz.cc
+++ b/tests/fuzz/crypt2_load_fuzz.cc
@@ -107,9 +107,7 @@ int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
 		err(EXIT_FAILURE, "mkostemp() failed");
 
 	/* enlarge header */
-	if (lseek(fd, FILESIZE-1, SEEK_SET) == -1 ||
-	    write(fd, "\0", 1) < 1 ||
-	    lseek(fd, 0, SEEK_SET) == -1)
+	if (ftruncate(fd, FILESIZE) == -1)
 		goto out;
 
 	if (write_buffer(fd, data, size) != (ssize_t)size)
diff --git a/tests/fuzz/plain_json_proto_to_luks2_converter.cc b/tests/fuzz/plain_json_proto_to_luks2_converter.cc
index cbf7bef0..c6de05a7 100644
--- a/tests/fuzz/plain_json_proto_to_luks2_converter.cc
+++ b/tests/fuzz/plain_json_proto_to_luks2_converter.cc
@@ -123,13 +123,9 @@ void LUKS2ProtoConverter::convert(const LUKS2_both_headers &headers, int fd) {
   if (!write_headers_only)
     out_size += KEYSLOTS_SIZE + DATA_SIZE;
 
-  result = lseek(fd, out_size - 1, SEEK_SET);
+  result = ftruncate(fd, out_size);
   if (result == -1)
-    err(EXIT_FAILURE, "lseek failed");
-
-  result = write(fd, "\0", 1);
-  if (result != 1)
-    err(EXIT_FAILURE, "write failed");
+    err(EXIT_FAILURE, "truncate failed");
 
   result = lseek(fd, 0, SEEK_SET);
   if (result == -1)
diff --git a/tests/fuzz/proto_to_luks2_converter.cc b/tests/fuzz/proto_to_luks2_converter.cc
index 87dc1841..3f74898c 100644
--- a/tests/fuzz/proto_to_luks2_converter.cc
+++ b/tests/fuzz/proto_to_luks2_converter.cc
@@ -511,13 +511,9 @@ void LUKS2ProtoConverter::convert(const LUKS2_both_headers &headers, int fd) {
   if (!write_headers_only)
     out_size += KEYSLOTS_SIZE + DATA_SIZE;
 
-  result = lseek(fd, out_size - 1, SEEK_SET);
+  result = ftruncate(fd, out_size);
   if (result == -1)
-    err(EXIT_FAILURE, "lseek failed");
-
-  result = write(fd, "\0", 1);
-  if (result != 1)
-    err(EXIT_FAILURE, "write failed");
+    err(EXIT_FAILURE, "truncate failed");
 
   result = lseek(fd, 0, SEEK_SET);
   if (result == -1)