diff --git a/src/lib-dcrypt/test-stream.c b/src/lib-dcrypt/test-stream.c
index aa5a7f48da..51913e9899 100644
--- a/src/lib-dcrypt/test-stream.c
+++ b/src/lib-dcrypt/test-stream.c
@@ -213,7 +213,7 @@ void test_write_read_v1(void)
 
 		test_assert_idx(pos + siz <= sizeof(payload), pos);
 		if (pos + siz > sizeof(payload)) break;
-		test_assert_idx(memcmp(ptr, payload + pos, siz) == 0, pos);
+		test_assert_idx(siz == 0 || memcmp(ptr, payload + pos, siz) == 0, pos);
 		i_stream_skip(is_2, siz); pos += siz;
 	}
 
@@ -264,7 +264,7 @@ void test_write_read_v1_short(void)
 
 		test_assert_idx(pos + siz <= sizeof(payload), pos);
 		if (pos + siz > sizeof(payload)) break;
-		test_assert_idx(memcmp(ptr, payload + pos, siz) == 0, pos);
+		test_assert_idx(siz == 0 || memcmp(ptr, payload + pos, siz) == 0, pos);
 		i_stream_skip(is_2, siz); pos += siz;
 	}
 
@@ -357,7 +357,7 @@ void test_write_read_v2(void)
 
 		test_assert_idx(pos + siz <= sizeof(payload), pos);
 		if (pos + siz > sizeof(payload)) break;
-		test_assert_idx(memcmp(ptr, payload + pos, siz) == 0, pos);
+		test_assert_idx(siz == 0 || memcmp(ptr, payload + pos, siz) == 0, pos);
 		i_stream_skip(is_2, siz); pos += siz;
 	}
 
@@ -405,7 +405,7 @@ void test_write_read_v2_short(void)
 
 		test_assert_idx(pos + siz <= sizeof(payload), pos);
 		if (pos + siz > sizeof(payload)) break;
-		test_assert_idx(memcmp(ptr, payload + pos, siz) == 0, pos);
+		test_assert_idx(siz == 0 || memcmp(ptr, payload + pos, siz) == 0, pos);
 		i_stream_skip(is_2, siz); pos += siz;
 	}
 
diff --git a/src/lib-index/mail-cache-lookup.c b/src/lib-index/mail-cache-lookup.c
index 5acc59f97b..ee8b378c41 100644
--- a/src/lib-index/mail-cache-lookup.c
+++ b/src/lib-index/mail-cache-lookup.c
@@ -477,9 +477,11 @@ static void header_lines_save(struct header_lookup_context *ctx,
 
 	hdr_data = p_new(ctx->pool, struct header_lookup_data, 1);
 	hdr_data->data_size = data_size;
-	hdr_data->data = data_dup = data_size == 0 ? NULL :
-		p_malloc(ctx->pool, data_size);
-	memcpy(data_dup, CONST_PTR_OFFSET(field->data, pos), data_size);
+	if (data_size > 0) {
+		hdr_data->data = data_dup =
+			p_malloc(ctx->pool, data_size);
+		memcpy(data_dup, CONST_PTR_OFFSET(field->data, pos), data_size);
+	}
 
 	for (i = 0; i < lines_count; i++) {
 		hdr_line.line_num = lines[i];
diff --git a/src/lib-storage/index/dbox-multi/mdbox-storage.c b/src/lib-storage/index/dbox-multi/mdbox-storage.c
index 7e0b176f68..0a61cb8680 100644
--- a/src/lib-storage/index/dbox-multi/mdbox-storage.c
+++ b/src/lib-storage/index/dbox-multi/mdbox-storage.c
@@ -214,7 +214,8 @@ int mdbox_read_header(struct mdbox_mailbox *mbox,
 		return -1;
 	}
 	memset(hdr, 0, sizeof(*hdr));
-	memcpy(hdr, data, I_MIN(data_size, sizeof(*hdr)));
+	if (data_size > 0)
+		memcpy(hdr, data, I_MIN(data_size, sizeof(*hdr)));
 	*need_resize_r = data_size < sizeof(*hdr);
 	return 0;
 }
diff --git a/src/lib-storage/index/index-mailbox-size.c b/src/lib-storage/index/index-mailbox-size.c
index 1fd4008dfd..0153a094b4 100644
--- a/src/lib-storage/index/index-mailbox-size.c
+++ b/src/lib-storage/index/index-mailbox-size.c
@@ -57,8 +57,10 @@ static void vsize_header_refresh(struct mailbox_vsize_update *update)
 
 	mail_index_get_header_ext(update->view, update->box->vsize_hdr_ext_id,
 				  &data, &size);
-	memcpy(&update->orig_vsize_hdr, data,
-	       I_MIN(size, sizeof(update->orig_vsize_hdr)));
+	if (size > 0) {
+		memcpy(&update->orig_vsize_hdr, data,
+		       I_MIN(size, sizeof(update->orig_vsize_hdr)));
+	}
 	if (size == sizeof(update->vsize_hdr))
 		memcpy(&update->vsize_hdr, data, sizeof(update->vsize_hdr));
 	else {
diff --git a/src/lib/buffer.c b/src/lib/buffer.c
index 1a536a51d1..a350de14ee 100644
--- a/src/lib/buffer.c
+++ b/src/lib/buffer.c
@@ -185,7 +185,8 @@ void buffer_write(buffer_t *_buf, size_t pos,
 	struct real_buffer *buf = (struct real_buffer *)_buf;
 
 	buffer_check_limits(buf, pos, data_size);
-	memcpy(buf->w_buffer + pos, data, data_size);
+	if (data_size > 0)
+		memcpy(buf->w_buffer + pos, data, data_size);
 }
 
 void buffer_append(buffer_t *buf, const void *data, size_t data_size)
diff --git a/src/lib/istream.c b/src/lib/istream.c
index 4727e87477..80ad7d221f 100644
--- a/src/lib/istream.c
+++ b/src/lib/istream.c
@@ -593,8 +593,10 @@ int i_stream_read_data(struct istream *stream, const unsigned char **data_r,
 
 void i_stream_compress(struct istream_private *stream)
 {
-	memmove(stream->w_buffer, stream->w_buffer + stream->skip,
-		stream->pos - stream->skip);
+	if (stream->skip != stream->pos) {
+		memmove(stream->w_buffer, stream->w_buffer + stream->skip,
+			stream->pos - stream->skip);
+	}
 	stream->pos -= stream->skip;
 
 	stream->skip = 0;