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diff --git a/port.c b/port.c
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index 4fef7fa..138682a 100644
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--- a/port.c
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+++ b/port.c
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@@ -126,7 +126,7 @@ macaroon_secretbox_open(const unsigned char* enc_key,
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 void
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 macaroon_bin2hex(const unsigned char* bin, size_t bin_sz, char* hex)
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 {
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-    void* ptr = sodium_bin2hex(hex, bin_sz * 2, bin, bin_sz);
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+    void* ptr = sodium_bin2hex(hex, bin_sz * 2 + 1, bin, bin_sz);
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     assert(ptr == hex);
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 }
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