diff --git a/.travis.yml b/.travis.yml
index 05faa33a..0c227c7c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,8 +10,8 @@ compiler:
 # Settings
 env:
   - CTEST_OUTPUT_ON_FAILURE=1 OPTIONS="-DVMIME_SENDMAIL_PATH=/path/to/sendmail -DCMAKE_BUILD_TYPE=Debug -DVMIME_BUILD_TESTS=ON"
-  - CTEST_OUTPUT_ON_FAILURE=1 OPTIONS="-DVMIME_SENDMAIL_PATH=/path/to/sendmail -DCMAKE_BUILD_TYPE=Debug -DVMIME_BUILD_TESTS=ON -DVMIME_CHARSETCONV_LIB_IS_ICU=YES"
-  - CTEST_OUTPUT_ON_FAILURE=1 OPTIONS="-DVMIME_SENDMAIL_PATH=/path/to/sendmail -DCMAKE_BUILD_TYPE=Debug -DVMIME_BUILD_TESTS=ON -DVMIME_TLS_SUPPORT_LIB_IS_OPENSSL=YES"
+  - CTEST_OUTPUT_ON_FAILURE=1 OPTIONS="-DVMIME_SENDMAIL_PATH=/path/to/sendmail -DCMAKE_BUILD_TYPE=Debug -DVMIME_BUILD_TESTS=ON -DVMIME_CHARSETCONV_LIB_IS_ICONV=OFF -DVMIME_CHARSETCONV_LIB_IS_ICU=ON"
+  - CTEST_OUTPUT_ON_FAILURE=1 OPTIONS="-DVMIME_SENDMAIL_PATH=/path/to/sendmail -DCMAKE_BUILD_TYPE=Debug -DVMIME_BUILD_TESTS=ON -DVMIME_TLS_SUPPORT_LIB_IS_GNUTLS=OFF -DVMIME_TLS_SUPPORT_LIB_IS_OPENSSL=ON"
 
 # Make sure some required tools/libraries are installed
 install:
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6809e7c3..1c45d892 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -574,6 +574,10 @@ OPTION(
 
 IF(VMIME_HAVE_TLS_SUPPORT)
 
+	IF(VMIME_TLS_SUPPORT_LIB_IS_GNUTLS AND VMIME_TLS_SUPPORT_LIB_IS_OPENSSL)
+		MESSAGE(FATAL_ERROR "Options VMIME_TLS_SUPPORT_LIB_IS_GNUTLS and VMIME_TLS_SUPPORT_LIB_IS_OPENSSL are mutually exclusive (select one or the other, but not both!)")
+	ENDIF()
+
 	IF(VMIME_TLS_SUPPORT_LIB_IS_GNUTLS)
 
 		INCLUDE_DIRECTORIES(
@@ -650,6 +654,10 @@ OPTION(
 	${VMIME_CHARSETCONV_LIB_IS_ICU_DEFAULT}
 )
 
+IF(VMIME_CHARSETCONV_LIB_IS_ICONV AND VMIME_CHARSETCONV_LIB_IS_ICU)
+	MESSAGE(FATAL_ERROR "Options VMIME_CHARSETCONV_LIB_IS_ICONV and VMIME_CHARSETCONV_LIB_IS_ICU are mutually exclusive (select one or the other, but not both!)")
+ENDIF()
+
 IF(VMIME_CHARSETCONV_LIB_IS_ICONV)
 
 	INCLUDE_DIRECTORIES(