diff --git a/include/mbedtls/entropy.h b/include/mbedtls/entropy.h
index cee6ab5..41648aa 100644
--- a/include/mbedtls/entropy.h
+++ b/include/mbedtls/entropy.h
@@ -147,13 +147,15 @@
  */
 typedef struct
 {
-    int accumulator_started;
+    int accumulator_started; /* 0 after init.
+                              * 1 after the first update.
+                              * -1 after free. */
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
     mbedtls_sha512_context  accumulator;
 #else
     mbedtls_sha256_context  accumulator;
 #endif
-    int             source_count;
+    int             source_count; /* Number of entries used in source. */
     mbedtls_entropy_source_state    source[MBEDTLS_ENTROPY_MAX_SOURCES];
 #if defined(MBEDTLS_HAVEGE_C)
     mbedtls_havege_state    havege_data;