diff --git a/sknw/sknw.py b/sknw/sknw.py
index bb5e838..99bc1f7 100644
--- a/sknw/sknw.py
+++ b/sknw/sknw.py
@@ -25,7 +25,7 @@ def mark(img, nbs): # mark the array use (0, 1, 2)
 
 @jit(nopython=True) # trans index to r, c...
 def idx2rc(idx, acc):
-    rst = np.zeros((len(idx), len(acc)), dtype=np.int16)
+    rst = np.zeros((len(idx), len(acc)), dtype=np.int64)
     for i in range(len(idx)):
         for j in range(len(acc)):
             rst[i,j] = idx[i]//acc[j]
@@ -57,6 +57,8 @@ def trace(img, p, nbs, acc, buf):
     c1 = 0; c2 = 0;
     newp = 0
     cur = 1
+    buf_len = len(buf)
+    buf_overflow = False
     while True:
         buf[cur] = p
         img[p] = 0
@@ -74,12 +76,15 @@ def trace(img, p, nbs, acc, buf):
                 newp = cp
         p = newp
         if c2!=0:break
-    return (c1-10, c2-10, idx2rc(buf[:cur+1], acc))
+        if cur >= buf_len:
+            buf_overflow = True
+            break
+    return (c1-10, c2-10, idx2rc(buf[:cur+1], acc)), buf_overflow
    
 @jit(nopython=True) # parse the image then get the nodes and edges
-def parse_struc(img, nbs, acc, iso, ring):
+def parse_struc(img, nbs, acc, iso, ring, buf_size):
     img = img.ravel()
-    buf = np.zeros(131072, dtype=np.int64)
+    buf = np.zeros(buf_size, dtype=np.int64)
     num = 10
     nodes = []
     for p in range(len(img)):
@@ -93,23 +98,27 @@ def parse_struc(img, nbs, acc, iso, ring):
         if img[p] <10: continue
         for dp in nbs:
             if img[p+dp]==1:
-                edge = trace(img, p+dp, nbs, acc, buf)
+                edge, buf_overflow = trace(img, p+dp, nbs, acc, buf)
+                if buf_overflow:
+                    return nodes, edges, buf_overflow
                 edges.append(edge)
-    if not ring: return nodes, edges
+    if not ring: return nodes, edges, False
     for p in range(len(img)):
         if img[p]!=1: continue
         img[p] = num; num += 1
         nodes.append(idx2rc([p], acc))
         for dp in nbs:
             if img[p+dp]==1:
-                edge = trace(img, p+dp, nbs, acc, buf)
+                edge, buf_overflow = trace(img, p+dp, nbs, acc, buf)
+                if buf_overflow:
+                    return nodes, edges, buf_overflow
                 edges.append(edge)
-    return nodes, edges
+    return nodes, edges, False
     
 # use nodes and edges build a networkx graph
 def build_graph(nodes, edges, multi=False, full=True):
     os = np.array([i.mean(axis=0) for i in nodes])
-    if full: os = os.round().astype(np.uint16)
+    if full: os = os.round().astype(np.uint32)
     graph = nx.MultiGraph() if multi else nx.Graph()
     for i in range(len(nodes)):
         graph.add_node(i, pts=nodes[i], o=os[i])
@@ -126,12 +135,14 @@ def mark_node(ske):
     mark(buf, nbs)
     return buf
     
-def build_sknw(ske, multi=False, iso=True, ring=True, full=True):
-    buf = np.pad(ske, (1,1), mode='constant').astype(np.uint16)
+def build_sknw(ske, multi=False, iso=True, ring=True, full=True, buf_size=131072):
+    buf = np.pad(ske, (1,1), mode='constant').astype(np.int64)
     nbs = neighbors(buf.shape)
     acc = np.cumprod((1,)+buf.shape[::-1][:-1])[::-1]
     mark(buf, nbs)
-    nodes, edges = parse_struc(buf, nbs, acc, iso, ring)
+    nodes, edges, buf_overflow = parse_struc(buf, nbs, acc, iso, ring, buf_size)
+    if buf_overflow:
+        raise Exception('Buffer overflow')
     return build_graph(nodes, edges, multi, full)
     
 # draw the graph