diff --git a/include/villas/sample.h b/include/villas/sample.h
index d50fa11a9..163d28f2f 100644
--- a/include/villas/sample.h
+++ b/include/villas/sample.h
@@ -65,8 +65,8 @@ enum sample_flags {
 
 struct sample {
 	uint64_t sequence;	/**< The sequence number of this sample. */
-	int length;		/**< The number of values in sample::values which are valid. */
-	int capacity;		/**< The number of values in sample::values for which memory is reserved. */
+	unsigned length;	/**< The number of values in sample::values which are valid. */
+	unsigned capacity;	/**< The number of values in sample::values for which memory is reserved. */
 	int flags;		/**< Flags are used to store binary properties of a sample. */
 
 	struct vlist *signals;	/**< The list of signal descriptors. */