diff --git a/shell_wrapper/kahe.cc b/shell_wrapper/kahe.cc
index 2fc9583..8c1d3b7 100644
--- a/shell_wrapper/kahe.cc
+++ b/shell_wrapper/kahe.cc
@@ -322,12 +322,6 @@ FfiStatus PackMessagesRaw(rust::Slice<const uint64_t> messages,
     return MakeFfiStatus(absl::InvalidArgumentError(
         secure_aggregation::kNullPointerErrorMessage));
   }
-
-  // Allocate the vector for output packed values if needed.
-  if (packed_values->ptr == nullptr) {
-    packed_values->ptr =
-        std::make_unique<std::vector<secure_aggregation::BigInteger>>();
-  }
   auto curr_packed_values =
       rlwe::PackMessagesFlat<secure_aggregation::Integer,
                              secure_aggregation::BigInteger>(
@@ -339,6 +333,11 @@ FfiStatus PackMessagesRaw(rust::Slice<const uint64_t> messages,
   }
   // Pad with zeros if needed.
   curr_packed_values.resize(num_packed_values, 0);
+  // Allocate the vector for output packed values if needed.
+  if (packed_values->ptr == nullptr) {
+    packed_values->ptr =
+        std::make_unique<std::vector<secure_aggregation::BigInteger>>();
+  }
   // Append the packed values to the end of the output vector.
   packed_values->ptr->insert(packed_values->ptr->end(),
                              curr_packed_values.begin(),
diff --git a/shell_wrapper/kahe.rs b/shell_wrapper/kahe.rs
index 40c6a73..699a455 100644
--- a/shell_wrapper/kahe.rs
+++ b/shell_wrapper/kahe.rs
@@ -37,7 +37,7 @@ mod ffi {
     }
 
     pub struct BigIntVectorWrapper {
-        pub ptr: UniquePtr<CxxVector<BigInteger>>,
+        pub(crate) ptr: UniquePtr<CxxVector<BigInteger>>,
     }
 
     unsafe extern "C++" {
@@ -190,7 +190,11 @@ pub fn encrypt(
     params: &KahePublicParametersWrapper,
     prng: &mut SingleThreadHkdfWrapper,
 ) -> Result<RnsPolynomialVec, status::StatusError> {
-    let mut packed_values = MaybeUninit::<BigIntVectorWrapper>::zeroed();
+    // SAFETY: this initializes `packed_values` with packed_values.ptr == nullptr. The following
+    // loop ensures that we either return an error (and drop `packed_values`, including potential
+    // partial allocations), or make packed_values.ptr point to a valid C++ vector.
+    let mut packed_values = BigIntVectorWrapper { ptr: cxx::UniquePtr::null() };
+
     // SAFETY: No lifetime constraints (`PackMessagesRaw` may create a new vector of BigIntegers
     // wrapped by `packed_values` which does not keep any reference to the inputs).
     // `PackMessagesRaw` only appends to the C++ vector wrapped by `packed_values`,
@@ -206,7 +210,7 @@ pub fn encrypt(
                 packed_vector_config.base,
                 packed_vector_config.dimension,
                 packed_vector_config.num_packed_coeffs,
-                packed_values.as_mut_ptr(),
+                &mut packed_values,
             )
         })?;
     }
@@ -217,7 +221,7 @@ pub fn encrypt(
     // wrapped by `packed_values`, updates the states wrapped by `prng`, and writes into the C++
     // vector wrapped by `out`.
     rust_status_from_cpp(unsafe {
-        ffi::Encrypt(&packed_values.assume_init(), secret_key, params, prng, out.as_mut_ptr())
+        ffi::Encrypt(&packed_values, secret_key, params, prng, out.as_mut_ptr())
     })?;
     // SAFETY: `out` is safely initialized if we get to this point.
     Ok(unsafe { out.assume_init() })
@@ -239,6 +243,9 @@ pub fn decrypt(
         ffi::Decrypt(ciphertext, secret_key, params, packed_values.as_mut_ptr())
     })?;
 
+    // SAFETY: `packed_values` is safely initialized if we get to this point.
+    let mut packed_values = unsafe { packed_values.assume_init() };
+
     let mut output_vectors = HashMap::<String, Vec<u64>>::new();
     // Assume the packed values are stored in the same order as the configs.
     for (id, packed_vector_config) in packed_vector_configs.iter() {
@@ -253,7 +260,7 @@ pub fn decrypt(
                 packed_vector_config.base,
                 packed_vector_config.dimension,
                 packed_vector_config.num_packed_coeffs,
-                packed_values.assume_init_mut(),
+                &mut packed_values,
                 &mut unpacked_values,
             )
         })?;