diff --git a/include/librustzcash.h b/include/librustzcash.h
index b48136b..f5d7d1e 100644
--- a/include/librustzcash.h
+++ b/include/librustzcash.h
@@ -112,7 +112,7 @@ extern "C" {
     /// signature. You must provide the intended valueBalance so that
     /// we can internally check consistency.
     bool librustzcash_sapling_binding_sig(
-        void *ctx,
+        const void *ctx,
         int64_t valueBalance,
         const unsigned char *sighash,
         unsigned char *result
diff --git a/src/rustzcash.rs b/src/rustzcash.rs
index 93e8f1b..5eb1dcf 100644
--- a/src/rustzcash.rs
+++ b/src/rustzcash.rs
@@ -1188,7 +1188,7 @@ pub extern "system" fn librustzcash_sapling_spend_sig(
 
 #[no_mangle]
 pub extern "system" fn librustzcash_sapling_binding_sig(
-    ctx: *mut SaplingProvingContext,
+    ctx: *const SaplingProvingContext,
     value_balance: int64_t,
     sighash: *const [c_uchar; 32],
     result: *mut [c_uchar; 64],