diff --git a/sigpy/wavelet.py b/sigpy/wavelet.py
index ea1cc60f..82fcf26d 100644
--- a/sigpy/wavelet.py
+++ b/sigpy/wavelet.py
@@ -147,22 +147,25 @@ def iwt(input, oshape, wave_name='db4'):
     pad_shape = [(oshape[k] + rec_hi.size - 1 + (oshape[k] + rec_hi.size - 1) % 2) if k in axes else oshape[k] \
                     for k in range(len(oshape))]
     
+    inputdct = {}
     approxdct = {}
     for key in list(input.keys()):
         if (int(key[:4]) < max_level):
-            approxdct[key] = input.pop(key)
+            approxdct[key] = input[key]
+        else:
+            inputdct[key] = input[key]
 
     if approxdct != {}:
         approxkey = [elm for elm in approxdct.keys() if 'H' not in elm].pop()
         approxkey = "%04d%s" % (max_level, approxkey[4:])
         ashape = [pad_shape[k]//2 if k in axes else oshape[k] for k in range(len(oshape))]
-        input[approxkey] = iwt(approxdct, ashape, wave_name=wave_name)
+        inputdct[approxkey] = iwt(approxdct, ashape, wave_name=wave_name)
 
     res = xp.zeros(oshape)
     X   = xp.zeros(pad_shape).astype(xp.complex64)
     f   = xp.ones(pad_shape).astype(xp.complex64)
-    for key in input.keys():
-        X[sampleidx] = input[key];
+    for key in inputdct.keys():
+        X[sampleidx] = inputdct[key];
         X = xp.fft.fftn(X, axes=axes)
         subkeys = list(key)[4:]
         for k in range(len(subkeys)):