diff --git a/README.md b/README.md
index e514607..2160e74 100644
--- a/README.md
+++ b/README.md
@@ -117,7 +117,7 @@ TINYEXPR defines only five functions:
 ```C
     double te_interp(const char *expression, int *error);
     te_expr *te_compile(const char *expression, const te_variable *lookup, int lookup_len, int *error);
-    double te_eval(te_expr *n);
+    double te_eval(const te_expr *n);
     void te_print(const te_expr *n);
     void te_free(te_expr *n);
 ```
diff --git a/tinyexpr.c b/tinyexpr.c
index 3c4e95d..8364ef1 100644
--- a/tinyexpr.c
+++ b/tinyexpr.c
@@ -306,7 +306,7 @@ static te_expr *expr(state *s) {
 }
 
 
-double te_eval(te_expr *n) {
+double te_eval(const te_expr *n) {
     double ret;
 
     if (n->bound) {
diff --git a/tinyexpr.h b/tinyexpr.h
index 1df74d2..45da464 100644
--- a/tinyexpr.h
+++ b/tinyexpr.h
@@ -61,7 +61,7 @@ double te_interp(const char *expression, int *error);
 te_expr *te_compile(const char *expression, const te_variable *lookup, int lookup_len, int *error);
 
 /* Evaluates the expression. */
-double te_eval(te_expr *n);
+double te_eval(const te_expr *n);
 
 /* Prints debugging information on the syntax tree. */
 void te_print(const te_expr *n);