diff --git a/src/common/ossl_helpers.c b/src/common/ossl_helpers.c
index 85f6718b8..d2b2d99e9 100644
--- a/src/common/ossl_helpers.c
+++ b/src/common/ossl_helpers.c
@@ -46,30 +46,58 @@ static void fetch_ossl_objects(void) {
 }
 
 static void free_ossl_objects(void) {
-	OSSL_FUNC(EVP_MD_free)(sha256_ptr);
-	sha256_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(sha384_ptr);
-	sha384_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(sha512_ptr);
-	sha512_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(sha3_256_ptr);
-	sha3_256_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(sha3_384_ptr);
-	sha3_384_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(sha3_512_ptr);
-	sha3_512_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(shake128_ptr);
-	shake128_ptr = NULL;
-	OSSL_FUNC(EVP_MD_free)(shake256_ptr);
-	shake256_ptr = NULL;
-	OSSL_FUNC(EVP_CIPHER_free)(aes128_ecb_ptr);
-	aes128_ecb_ptr = NULL;
-	OSSL_FUNC(EVP_CIPHER_free)(aes128_ctr_ptr);
-	aes128_ctr_ptr = NULL;
-	OSSL_FUNC(EVP_CIPHER_free)(aes256_ecb_ptr);
-	aes256_ecb_ptr = NULL;
-	OSSL_FUNC(EVP_CIPHER_free)(aes256_ctr_ptr);
-	aes256_ctr_ptr = NULL;
+	/* Always check argument is non-NULL before calling EVP_*_free
+	 * to avoid OpenSSL functions being used when they are
+	 * overridden with OQS_*_set_callbacks.
+	 */
+	if (sha256_ptr) {
+		OSSL_FUNC(EVP_MD_free)(sha256_ptr);
+		sha256_ptr = NULL;
+	}
+	if (sha384_ptr) {
+		OSSL_FUNC(EVP_MD_free)(sha384_ptr);
+		sha384_ptr = NULL;
+	}
+	if (sha512_ptr) {
+		OSSL_FUNC(EVP_MD_free)(sha512_ptr);
+		sha512_ptr = NULL;
+	}
+	if (sha3_256_ptr) {
+		OSSL_FUNC(EVP_MD_free)(sha3_256_ptr);
+		sha3_256_ptr = NULL;
+	}
+	if (sha3_384_ptr) {
+		OSSL_FUNC(EVP_MD_free)(sha3_384_ptr);
+		sha3_384_ptr = NULL;
+	}
+	if (sha3_512_ptr) {
+		OSSL_FUNC(EVP_MD_free)(sha3_512_ptr);
+		sha3_512_ptr = NULL;
+	}
+	if (shake128_ptr) {
+		OSSL_FUNC(EVP_MD_free)(shake128_ptr);
+		shake128_ptr = NULL;
+	}
+	if (shake256_ptr) {
+		OSSL_FUNC(EVP_MD_free)(shake256_ptr);
+		shake256_ptr = NULL;
+	}
+	if (aes128_ecb_ptr) {
+		OSSL_FUNC(EVP_CIPHER_free)(aes128_ecb_ptr);
+		aes128_ecb_ptr = NULL;
+	}
+	if (aes128_ctr_ptr) {
+		OSSL_FUNC(EVP_CIPHER_free)(aes128_ctr_ptr);
+		aes128_ctr_ptr = NULL;
+	}
+	if (aes256_ecb_ptr) {
+		OSSL_FUNC(EVP_CIPHER_free)(aes256_ecb_ptr);
+		aes256_ecb_ptr = NULL;
+	}
+	if (aes256_ctr_ptr) {
+		OSSL_FUNC(EVP_CIPHER_free)(aes256_ctr_ptr);
+		aes256_ctr_ptr = NULL;
+	}
 }
 #endif // OPENSSL_VERSION_NUMBER >= 0x30000000L