diff --git a/include/ntlmclient.h b/include/ntlmclient.h
index d109a5c..2923662 100644
--- a/include/ntlmclient.h
+++ b/include/ntlmclient.h
@@ -22,6 +22,19 @@ extern "C" {
 
 typedef struct ntlm_client ntlm_client;
 
+typedef enum {
+	/**
+	 * An error occurred; more details are available by querying
+	 * `ntlm_client_errmsg`.
+	 */
+	NTLM_CLIENT_ERROR = -1,
+
+	/**
+	 * The input provided to the function is missing or invalid.
+	 */
+	NTLM_CLIENT_ERROR_INVALID_INPUT = -2,
+} ntlm_error_code;
+
 /*
  * Flags for initializing the `ntlm_client` context.  A combination of
  * these flags can be provided to `ntlm_client_init`.
diff --git a/src/ntlm.c b/src/ntlm.c
index 979d612..3393be9 100644
--- a/src/ntlm.c
+++ b/src/ntlm.c
@@ -9,7 +9,6 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
-#include <assert.h>
 #include <errno.h>
 #include <ctype.h>
 #include <unistd.h>
@@ -24,6 +23,18 @@
 #include "compat.h"
 #include "util.h"
 
+#define NTLM_ASSERT_ARG(expr) do { \
+		if (!(expr)) \
+			return NTLM_CLIENT_ERROR_INVALID_INPUT; \
+	} while(0)
+
+#define NTLM_ASSERT(ntlm, expr) do { \
+		if (!(expr)) { \
+			ntlm_client_set_errmsg(ntlm, "internal error: " #expr); \
+			return -1; \
+		} \
+	} while(0)
+
 unsigned char ntlm_client_signature[] = NTLM_SIGNATURE;
 
 static bool supports_unicode(ntlm_client *ntlm)
@@ -74,7 +85,9 @@ void ntlm_client_set_errmsg(ntlm_client *ntlm, const char *errmsg)
 
 const char *ntlm_client_errmsg(ntlm_client *ntlm)
 {
-	assert(ntlm);
+	if (!ntlm)
+		return "internal error";
+
 	return ntlm->errmsg ? ntlm->errmsg : "no error";
 }
 
@@ -84,7 +97,7 @@ int ntlm_client_set_version(
 	uint8_t minor,
 	uint16_t build)
 {
-	assert(ntlm);
+	NTLM_ASSERT_ARG(ntlm);
 
 	ntlm->host_version.major = major;
 	ntlm->host_version.minor = minor;
@@ -111,8 +124,7 @@ int ntlm_client_set_hostname(
 	const char *hostname,
 	const char *domain)
 {
-	assert(ntlm);
-
+	NTLM_ASSERT_ARG(ntlm);
 	ENSURE_INITIALIZED(ntlm);
 
 	free_hostname(ntlm);
@@ -168,8 +180,7 @@ int ntlm_client_set_credentials(
 	const char *domain,
 	const char *password)
 {
-	assert(ntlm);
-
+	NTLM_ASSERT_ARG(ntlm);
 	ENSURE_INITIALIZED(ntlm);
 
 	free_credentials(ntlm);
@@ -218,8 +229,7 @@ int ntlm_client_set_credentials(
 
 int ntlm_client_set_target(ntlm_client *ntlm, const char *target)
 {
-	assert(ntlm);
-
+	NTLM_ASSERT_ARG(ntlm);
 	ENSURE_INITIALIZED(ntlm);
 
 	free(ntlm->target);
@@ -248,14 +258,16 @@ int ntlm_client_set_target(ntlm_client *ntlm, const char *target)
 
 int ntlm_client_set_nonce(ntlm_client *ntlm, uint64_t nonce)
 {
-	assert(ntlm);
+	NTLM_ASSERT_ARG(ntlm);
+
 	ntlm->nonce = nonce;
 	return 0;
 }
 
 int ntlm_client_set_timestamp(ntlm_client *ntlm, uint64_t timestamp)
 {
-	assert(ntlm);
+	NTLM_ASSERT_ARG(ntlm);
+
 	ntlm->timestamp = timestamp;
 	return 0;
 }
@@ -601,7 +613,9 @@ int ntlm_client_negotiate(
 	size_t hostname_offset = 0;
 	uint32_t flags = 0;
 
-	assert(out && out_len && ntlm);
+	NTLM_ASSERT_ARG(out);
+	NTLM_ASSERT_ARG(out_len);
+	NTLM_ASSERT_ARG(ntlm);
 
 	*out = NULL;
 	*out_len = 0;
@@ -684,20 +698,22 @@ int ntlm_client_negotiate(
 		return -1;
 
 	if (hostname_len > 0) {
-		assert(hostname_offset == ntlm->negotiate.pos);
+		NTLM_ASSERT(ntlm, hostname_offset == ntlm->negotiate.pos);
+
 		if (!write_buf(ntlm, &ntlm->negotiate,
 			(const unsigned char *)ntlm->hostname, hostname_len))
 			return -1;
 	}
 
 	if (domain_len > 0) {
-		assert(domain_offset == ntlm->negotiate.pos);
+		NTLM_ASSERT(ntlm, domain_offset == ntlm->negotiate.pos);
+
 		if (!write_buf(ntlm, &ntlm->negotiate,
 			(const unsigned char *)ntlm->hostdomain, domain_len))
 			return -1;
 	}
 
-	assert(ntlm->negotiate.pos == ntlm->negotiate.len);
+	NTLM_ASSERT(ntlm, ntlm->negotiate.pos == ntlm->negotiate.len);
 
 	ntlm->state = NTLM_STATE_CHALLENGE;
 
@@ -719,7 +735,8 @@ int ntlm_client_set_challenge(
 	uint32_t name_offset, info_offset = 0;
 	bool unicode, has_target_info = false;
 
-	assert(ntlm && (challenge_msg || !challenge_msg_len));
+	NTLM_ASSERT_ARG(ntlm);
+	NTLM_ASSERT_ARG(challenge_msg || !challenge_msg_len);
 
 	ENSURE_INITIALIZED(ntlm);
 
@@ -1101,7 +1118,7 @@ static bool generate_ntlm2_hash(
 		return false;
 	}
 
-	assert(out_len == NTLM_NTLM2_HASH_LEN);
+	NTLM_ASSERT(ntlm, out_len == NTLM_NTLM2_HASH_LEN);
 	return true;
 }
 
@@ -1122,7 +1139,7 @@ static bool generate_ntlm2_challengehash(
 		return false;
 	}
 
-	assert(out_len == 16);
+	NTLM_ASSERT(ntlm, out_len == 16);
 	return true;
 }
 
@@ -1143,7 +1160,7 @@ static bool generate_lm2_response(ntlm_client *ntlm,
 		return false;
 	}
 
-	assert(lm2_len == 16);
+	NTLM_ASSERT(ntlm, lm2_len == 16);
 
 	memcpy(&ntlm->lm_response[0], lm2_challengehash, 16);
 	memcpy(&ntlm->lm_response[16], &local_nonce, 8);
@@ -1237,7 +1254,9 @@ int ntlm_client_response(
 	uint32_t flags = 0;
 	bool unicode;
 
-	assert(out && out_len && ntlm);
+	NTLM_ASSERT_ARG(out);
+	NTLM_ASSERT_ARG(out_len);
+	NTLM_ASSERT_ARG(ntlm);
 
 	ENSURE_INITIALIZED(ntlm);
 
@@ -1362,7 +1381,7 @@ int ntlm_client_response(
 		!write_buf(ntlm, &ntlm->response, session, session_len))
 		return -1;
 
-	assert(ntlm->response.pos == ntlm->response.len);
+	NTLM_ASSERT(ntlm, ntlm->response.pos == ntlm->response.len);
 
 	ntlm->state = NTLM_STATE_COMPLETE;
 
@@ -1374,7 +1393,8 @@ int ntlm_client_response(
 
 void ntlm_client_reset(ntlm_client *ntlm)
 {
-	assert(ntlm);
+	if (!ntlm)
+		return;
 
 	ntlm->state = NTLM_STATE_NEGOTIATE;
 
diff --git a/tests/inputs.c b/tests/inputs.c
index 276c9c9..7178877 100644
--- a/tests/inputs.c
+++ b/tests/inputs.c
@@ -1,5 +1,6 @@
 #include "clar.h"
 #include "ntlm.h"
+#include "ntlm_tests.h"
 
 static ntlm_client *ntlm;
 
@@ -13,6 +14,31 @@ void test_inputs__cleanup(void)
 	ntlm_client_free(ntlm);
 }
 
+void test_inputs__null(void)
+{
+	const unsigned char *msg;
+	size_t msg_len;
+
+	cl_assert(ntlm_client_errmsg(NULL) != NULL);
+
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_set_hostname(NULL, "hostname", "HOSTDOMAIN"));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_set_credentials(NULL, "user", "DOMAIN", "pass!"));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_set_target(NULL, "target"));
+
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_negotiate(NULL, &msg_len, ntlm));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_negotiate(&msg, NULL, ntlm));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_negotiate(&msg, &msg_len, NULL));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_negotiate(NULL, NULL, NULL));
+
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_set_challenge(NULL, (const unsigned char *)"foo", 3));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_set_challenge(ntlm, NULL, 3));
+
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_response(NULL, &msg_len, ntlm));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_response(&msg, NULL, ntlm));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_response(&msg, &msg_len, NULL));
+	cl_must_fail_with(NTLM_CLIENT_ERROR_INVALID_INPUT, ntlm_client_response(NULL, NULL, NULL));
+}
+
 void test_inputs__set_hostname(void)
 {
 	cl_must_pass(ntlm_client_set_hostname(ntlm, "hostname", "HOSTDOMAIN"));
diff --git a/tests/ntlm_tests.h b/tests/ntlm_tests.h
index 8ccec24..6344bc4 100644
--- a/tests/ntlm_tests.h
+++ b/tests/ntlm_tests.h
@@ -5,6 +5,10 @@
 #include "ntlm.h"
 #include "util.h"
 
+#define cl_must_fail_with_(val, expr, desc) clar__assert((expr) == (val), __FILE__, __LINE__, "Expected function call to fail with " #val ": " #expr, desc, 0)
+
+#define cl_must_fail_with(val, expr) cl_must_fail_with_(val, expr, NULL)
+
 #define cl_ntlm_pass(ntlm, expr) cl_ntlm_expect((ntlm), (expr), 0, __FILE__, __LINE__)
 
 #define cl_ntlm_expect(ntlm, expr, expected, file, line) do { \