diff --git a/include/tree_sitter/parser.h b/include/tree_sitter/parser.h
index d5eca277..ea0d6d2d 100644
--- a/include/tree_sitter/parser.h
+++ b/include/tree_sitter/parser.h
@@ -39,6 +39,7 @@ typedef struct {
 typedef struct {
     ts_input input;
     const char *current_chunk;
+    size_t current_chunk_end;
     size_t position;
     size_t token_end_position;
     size_t token_start_position;
@@ -53,12 +54,15 @@ static void ts_lex(ts_parser *parser);
 static const ts_symbol * ts_recover(ts_state state, ts_state *to_state, size_t *count);
 
 static ts_parser ts_parser_make(ts_input input) {
+    size_t bytes_read = 0;
+    const char *chunk = input.read_fn(input.data, &bytes_read);
     ts_parser result = {
         .input = input,
         .token_start_position = 0,
         .token_end_position = 0,
         .position = 0,
-        .current_chunk = input.read_fn(input.data),
+        .current_chunk = chunk,
+        .current_chunk_end = bytes_read,
         .lookahead_node = NULL,
         .prev_lookahead_node = NULL,
         .lex_state = 0,
@@ -140,11 +144,12 @@ static void ts_parser_reduce(ts_parser *parser, ts_symbol symbol, int immediate_
 }
 
 static void ts_parser_advance(ts_parser *parser) {
-    if (parser->current_chunk && parser->current_chunk[parser->position]) {
+    if (parser->position < parser->current_chunk_end) {
         parser->position++;
     } else {
-        parser->current_chunk = parser->input.read_fn(parser->input.data);
-        parser->position = 0;
+        size_t bytes_read = 0;
+        parser->current_chunk = parser->input.read_fn(parser->input.data, &bytes_read);
+        parser->current_chunk_end += bytes_read;
     }
 }
 
diff --git a/include/tree_sitter/runtime.h b/include/tree_sitter/runtime.h
index 604b98a4..6f3a4bca 100644
--- a/include/tree_sitter/runtime.h
+++ b/include/tree_sitter/runtime.h
@@ -4,13 +4,13 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
-    
+
 #include <stdlib.h>
 
 typedef int ts_symbol;
 static const ts_symbol ts_builtin_sym_error = -1;
 static const ts_symbol ts_builtin_sym_end = -2;
-    
+
 typedef struct ts_tree {
     ts_symbol symbol;
     size_t ref_count;
@@ -42,7 +42,7 @@ ts_tree ** ts_tree_children(const ts_tree *tree);
 
 typedef struct {
     void *data;
-    const char * (* read_fn)(void *data);
+    const char * (* read_fn)(void *data, size_t *bytes_read);
     int (* seek_fn)(void *data, size_t position);
     void (* release_fn)(void *data);
 } ts_input;
@@ -61,7 +61,7 @@ void ts_document_set_input(ts_document *, ts_input input);
 void ts_document_set_input_string(ts_document *, const char *text);
 const ts_tree * ts_document_tree(const ts_document *);
 const char * ts_document_string(const ts_document *);
-    
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/runtime/document.c b/src/runtime/document.c
index 9cce0c58..b5b31bdd 100644
--- a/src/runtime/document.c
+++ b/src/runtime/document.c
@@ -37,8 +37,9 @@ typedef struct {
     size_t length;
 } ts_string_input_data;
 
-const char * ts_string_input_read(void *d) {
+const char * ts_string_input_read(void *d, size_t *bytes_read) {
     ts_string_input_data *data = (ts_string_input_data *)d;
+    *bytes_read = data->length;
     return data->string + data->position;
 }