diff --git a/devel.but b/devel.but
index e95e1d2..75ddeb7 100644
--- a/devel.but
+++ b/devel.but
@@ -1020,9 +1020,11 @@ it may return \cw{NULL}. If it does this, it must also set
 \q{Solution not known for this puzzle}); that error message is not
 expected to be dynamically allocated.
 
-If this function \e{does} produce a solution, it returns a move
-string suitable for feeding to \cw{execute_move()}
-(\k{backend-execute-move}).
+If this function \e{does} produce a solution, it returns a move string
+suitable for feeding to \cw{execute_move()}
+(\k{backend-execute-move}). Like a (non-empty) string returned from
+\cw{interpret_move()}, the returned string should be dynamically
+allocated.
 
 \H{backend-drawing} Drawing the game graphics
 
diff --git a/filling.c b/filling.c
index 4cd4318..cad0733 100644
--- a/filling.c
+++ b/filling.c
@@ -975,7 +975,7 @@ static char *solve_game(game_state *state, game_state *currstate,
         if (!solver(state->board, w, h, &aux))
             *error = "Sorry, I couldn't find a solution";
     }
-    return aux;
+    return dupstr(aux);
 }
 
 /*****************************************************************************