diff --git a/src/liblzma/common/filter_encoder.c b/src/liblzma/common/filter_encoder.c
index f0762d9a..73f9e106 100644
--- a/src/liblzma/common/filter_encoder.c
+++ b/src/liblzma/common/filter_encoder.c
@@ -37,9 +37,12 @@ typedef struct {
 	uint64_t (*block_size)(const void *options);
 
 	/// Tells the size of the Filter Properties field. If options are
-	/// invalid, UINT32_MAX is returned. If this is NULL, props_size_fixed
-	/// is used.
+	/// invalid, LZMA_OPTIONS_ERROR is returned and size is set to
+	/// UINT32_MAX.
 	lzma_ret (*props_size_get)(uint32_t *size, const void *options);
+
+	/// Some filters will always have the same size Filter Properties
+	/// field. If props_size_get is NULL, this value is used.
 	uint32_t props_size_fixed;
 
 	/// Encodes Filter Properties.