diff --git a/interface/wx/radiobut.h b/interface/wx/radiobut.h
index 0257bc31ce..3ae7778152 100644
--- a/interface/wx/radiobut.h
+++ b/interface/wx/radiobut.h
@@ -125,6 +125,8 @@ public:
    /**
         Returns the first radio button of the @c wxRB_GROUP this instance is in.
 
+        The return value is NULL if this button has the style @c wxRB_SINGLE.
+
         @see GetPreviousInGroup(), GetNextInGroup(), GetLastInGroup()
     */
     wxRadioButton* GetFirstInGroup();
@@ -132,6 +134,8 @@ public:
    /**
         Returns the last radio button of the @c wxRB_GROUP this instance is in.
 
+        The return value is NULL if this button has the style @c wxRB_SINGLE.
+
         @see GetFirstInGroup(), GetPreviousInGroup(), GetNextInGroup()
     */
     wxRadioButton* GetLastInGroup();
@@ -139,6 +143,9 @@ public:
    /**
         Returns the previous radio button of the @c wxRB_GROUP this instance is in.
 
+        The return value is NULL if there is no predecessor or this button has
+        the style @c wxRB_SINGLE.
+
         @see GetFirstInGroup(), GetNextInGroup(), GetLastInGroup()
     */
     wxRadioButton* GetPreviousInGroup();
@@ -146,6 +153,9 @@ public:
    /**
         Returns the next radio button of the @c wxRB_GROUP this instance is in.
 
+        The return value is NULL if there is no successor or this button has
+        the style @c wxRB_SINGLE.
+
         @see GetFirstInGroup(), GetPreviousInGroup(), GetLastInGroup()
     */
     wxRadioButton* GetNextInGroup();