diff --git a/src/ast/seq_decl_plugin.h b/src/ast/seq_decl_plugin.h
index 01f88112b..48d46923f 100644
--- a/src/ast/seq_decl_plugin.h
+++ b/src/ast/seq_decl_plugin.h
@@ -185,7 +185,7 @@ public:
 
     expr * get_some_value(sort * s) override;
 
-    bool is_char(ast* a) const { return a == m_char; }
+    bool is_char(sort* a) const { return a == m_char; }
 
     unsigned max_char() const { return get_char_plugin().max_char(); }
     unsigned num_bits() const { return get_char_plugin().num_bits(); }
@@ -222,7 +222,7 @@ public:
     sort* mk_string_sort() const { return seq.string_sort(); }
 
     bool is_char(sort* s) const { return seq.is_char(s); }
-    bool is_string(sort* s) const { return is_seq(s) && seq.is_char(s->get_parameter(0).get_ast()); }
+    bool is_string(sort* s) const { return is_seq(s) && seq.is_char(to_sort(s->get_parameter(0).get_ast())); }
     bool is_seq(sort* s) const { return is_sort_of(s, m_fid, SEQ_SORT); }
     bool is_re(sort* s) const { return is_sort_of(s, m_fid, RE_SORT); }
     bool is_re(sort* s, sort*& seq) const { return is_sort_of(s, m_fid, RE_SORT)  && (seq = to_sort(s->get_parameter(0).get_ast()), true); }