diff --git a/Makefile b/Makefile
index 1950016..7995b28 100644
--- a/Makefile
+++ b/Makefile
@@ -24,9 +24,9 @@ JS_PRE := $(wildcard javascript/*pre.js)
 JS_POST := $(wildcard javascript/*post.js)
 
 CPPFLAGS += -Iinclude -Ilib
-CFLAGS += -Wall -std=c89 -fPIC
-CXXFLAGS += -Wall -std=c++11 -fPIC
-LDFLAGS += -Wall
+CFLAGS += -Wall -Werror -std=c89 -fPIC
+CXXFLAGS += -Wall -Werror -std=c++11 -fPIC
+LDFLAGS += -Wall -Werror
 
 EMCCFLAGS = --closure 1 --memory-init-file 0 -s NO_FILESYSTEM=1 -s INVOKE_RUN=0
 # NO_BROWSER is kept for compatibility with emscripten 1.35.24, but is no
diff --git a/src/message.cpp b/src/message.cpp
index ff03903..e3c6f6b 100644
--- a/src/message.cpp
+++ b/src/message.cpp
@@ -136,7 +136,7 @@ static std::uint8_t const * decode(
         std::uint8_t const * len_start = pos;
         pos = varint_skip(pos, end);
         std::size_t len = varint_decode<std::size_t>(len_start, pos);
-        if (len + pos > end) return end;
+        if (len > std::size_t(end - pos)) return end;
         value = pos;
         value_length = len;
         pos += len;
@@ -157,7 +157,7 @@ static std::uint8_t const * skip_unknown(
             std::uint8_t const * len_start = pos;
             pos = varint_skip(pos, end);
             std::size_t len = varint_decode<std::size_t>(len_start, pos);
-            if (len + pos > end) return end;
+            if (len > std::size_t(end - pos)) return end;
             pos += len;
         } else {
             return end;
diff --git a/src/olm.cpp b/src/olm.cpp
index afbbbc6..69e02b9 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -191,7 +191,7 @@ const char * olm_account_last_error(
     OlmAccount * account
 ) {
     unsigned error = unsigned(from_c(account)->last_error);
-    if (error < sizeof(ERRORS)) {
+    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
         return ERRORS[error];
     } else {
         return "UNKNOWN_ERROR";
@@ -203,7 +203,7 @@ const char * olm_session_last_error(
     OlmSession * session
 ) {
     unsigned error = unsigned(from_c(session)->last_error);
-    if (error < sizeof(ERRORS)) {
+    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
         return ERRORS[error];
     } else {
         return "UNKNOWN_ERROR";
@@ -214,7 +214,7 @@ const char * olm_utility_last_error(
     OlmUtility * utility
 ) {
     unsigned error = unsigned(from_c(utility)->last_error);
-    if (error < sizeof(ERRORS)) {
+    if (error < (sizeof(ERRORS)/sizeof(ERRORS[0]))) {
         return ERRORS[error];
     } else {
         return "UNKNOWN_ERROR";
diff --git a/tests/test_olm_decrypt.cpp b/tests/test_olm_decrypt.cpp
index 2a2db98..4ec873c 100644
--- a/tests/test_olm_decrypt.cpp
+++ b/tests/test_olm_decrypt.cpp
@@ -67,7 +67,7 @@ int main() {
 {
 TestCase my_test("Olm decrypt test");
 
-for (int i = 0; i < sizeof(test_cases)/ sizeof(const char *); ++i) {
+for (unsigned int i = 0; i < sizeof(test_cases)/ sizeof(const char *); ++i) {
     decrypt_case(0, test_cases[i]);
 }
 
diff --git a/tests/test_ratchet.cpp b/tests/test_ratchet.cpp
index cbb3c52..8f89048 100644
--- a/tests/test_ratchet.cpp
+++ b/tests/test_ratchet.cpp
@@ -195,7 +195,7 @@ std::uint8_t random[] = "This is a random 32 byte string";
 for (unsigned i = 0; i < 8; ++i) {
 {
     std::uint8_t msg[alice.encrypt_output_length(sizeof(plaintext))];
-    std::uint8_t encrypt_length = alice.encrypt(
+    alice.encrypt(
         plaintext, 15, random, 32, msg, sizeof(msg)
     );
     std::uint8_t output[bob.decrypt_max_plaintext_length(msg, sizeof(msg))];
@@ -206,7 +206,7 @@ for (unsigned i = 0; i < 8; ++i) {
 random[31]++;
 {
     std::uint8_t msg[bob.encrypt_output_length(sizeof(plaintext))];
-    std::uint8_t encrypt_length = bob.encrypt(
+    bob.encrypt(
         plaintext, 15, random, 32, msg, sizeof(msg)
     );
     std::uint8_t output[alice.decrypt_max_plaintext_length(msg, sizeof(msg))];