diff --git a/DOCS/tech/slave.txt b/DOCS/tech/slave.txt
index be5cc06bbd..df0aa9a32f 100644
--- a/DOCS/tech/slave.txt
+++ b/DOCS/tech/slave.txt
@@ -400,7 +400,7 @@ stream_pos         pos       0               X   X        position in stream
 stream_start       pos       0               X            start pos in stream
 stream_end         pos       0               X            end pos in stream
 stream_length      pos       0               X            (end - start)
-length             double                    X            length of file in seconds
+length             time                      X            length of file in seconds
 metadata           str list                  X            list of metadata key/value
 metadata/*         string                    X            metadata values
 volume             float     0       100     X   X   X    change volume
diff --git a/command.c b/command.c
index 4fbeb1921e..98640cd477 100644
--- a/command.c
+++ b/command.c
@@ -1383,8 +1383,8 @@ static m_option_t mp_properties[] = {
      M_OPT_MIN, 0, 0, NULL },
     { "stream_length", mp_property_stream_length, CONF_TYPE_POSITION,
      M_OPT_MIN, 0, 0, NULL },
-    { "length", mp_property_length, CONF_TYPE_DOUBLE,
-     0, 0, 0, NULL },
+    { "length", mp_property_length, CONF_TYPE_TIME,
+     M_OPT_MIN, 0, 0, NULL },
     { "metadata", mp_property_metadata, CONF_TYPE_STRING_LIST,
      0, 0, 0, NULL },