diff --git a/c/flisp.c b/c/flisp.c
index 6245691..f94e135 100644
--- a/c/flisp.c
+++ b/c/flisp.c
@@ -261,12 +261,12 @@ SAFECAST_OP(string, char *, cvalue_data)
 
 struct symbol *symtab = NULL;
 
-int fl_is_keyword_name(char *str, size_t len)
+int fl_is_keyword_name(const char *str, size_t len)
 {
     return ((str[0] == ':' || str[len - 1] == ':') && str[1] != '\0');
 }
 
-static struct symbol *mk_symbol(char *str)
+static struct symbol *mk_symbol(const char *str)
 {
     struct symbol *sym;
     size_t len = strlen(str);
@@ -289,7 +289,7 @@ static struct symbol *mk_symbol(char *str)
     return sym;
 }
 
-static struct symbol **symtab_lookup(struct symbol **ptree, char *str)
+static struct symbol **symtab_lookup(struct symbol **ptree, const char *str)
 {
     int x;
 
@@ -305,7 +305,7 @@ static struct symbol **symtab_lookup(struct symbol **ptree, char *str)
     return ptree;
 }
 
-value_t symbol(char *str)
+value_t symbol(const char *str)
 {
     struct symbol **pnode;
 
diff --git a/c/flisp.h b/c/flisp.h
index a778d28..a00e359 100644
--- a/c/flisp.h
+++ b/c/flisp.h
@@ -141,9 +141,9 @@ extern value_t printprettysym, printreadablysym, printwidthsym;
 value_t fl_cons(value_t a, value_t b);
 value_t fl_list2(value_t a, value_t b);
 value_t fl_listn(size_t n, ...);
-value_t symbol(char *str);
+value_t symbol(const char *str);
 char *symbol_name(value_t v);
-int fl_is_keyword_name(char *str, size_t len);
+int fl_is_keyword_name(const char *str, size_t len);
 value_t alloc_vector(size_t n, int init);
 size_t llength(value_t v);
 value_t fl_compare(value_t a, value_t b);  // -1, 0, or 1