diff --git a/bash/NEWS.md b/bash/NEWS.md
index da7572c..3ce2966 100644
--- a/bash/NEWS.md
+++ b/bash/NEWS.md
@@ -36,6 +36,8 @@
 
  * `port4me --prepend=<ports>` would sort `<ports>`.
  
+ * `port4me --skip=<invalid>` would not given an error.
+ 
 
 # Version 0.6.0 [2023-07-13]
 
diff --git a/bash/incl/port4me.bash b/bash/incl/port4me.bash
index b0f43e4..a3b7c33 100644
--- a/bash/incl/port4me.bash
+++ b/bash/incl/port4me.bash
@@ -34,7 +34,7 @@
 #' Requirements:
 #' * Bash (>= 4)
 #'
-#' Version: 0.6.0-9013
+#' Version: 0.6.0-9015
 #' Copyright: Henrik Bengtsson (2022-2024)
 #' License: MIT
 #' Source code: https://github.com/HenrikBengtsson/port4me
@@ -63,6 +63,12 @@ _p4m_signal_error() {
     fi
 }
 
+_p4m_assert_integer() {
+    if grep -q -E "^[[:digit:]]+$" <<< "$*"; then
+        _p4m_error "Not an integer: $*"
+    fi
+}
+
 #' Check if TCP port can be opened
 #'
 #' Examples:
@@ -274,28 +280,27 @@ port4me() {
         _p4m_error "port4me requires Bash (>= 4): ${BASH_VERSION}. As a workaround, you could install the Python version (python -m pip port4me) and define a Bash function as: port4me() { python -m port4me \"\$@\"; }"
     fi
 
+    ## Validate arguments
     if [[ -n ${PORT4ME_TEST} ]]; then
+        if ! grep -q -E "^[[:digit:]]+$" <<< "${PORT4ME_TEST}"; then
+            _p4m_error "PORT4ME_TEST is not an integer: ${PORT4ME_TEST}"
+        fi
         tmp_int=${PORT4ME_TEST}
         if (( tmp_int < 1 || tmp_int > 65535 )); then
             _p4m_error "PORT4ME_TEST is out of range [1,65535]: ${tmp_int}"
         fi
     fi
     
+    ## Check port availability?
     if [[ $test -ne 0 ]]; then
         _p4m_can_port_be_opened "${test}"
         return $?
     fi
 
-    mapfile -t exclude < <(_p4m_parse_ports "${PORT4ME_EXCLUDE},${PORT4ME_EXCLUDE_SITE},${PORT4ME_EXCLUDE_UNSAFE}")
-    _p4m_signal_error "${exclude[@]}"
-
-    mapfile -t include < <(_p4m_parse_ports "${PORT4ME_INCLUDE},${PORT4ME_INCLUDE_SITE}")
-    _p4m_signal_error "${include[@]}"
-    
-    mapfile -t prepend < <(_p4m_parse_ports "${PORT4ME_PREPEND},${PORT4ME_PREPEND_SITE}" false)
-    _p4m_signal_error "${prepend[@]}"
-
     if [[ -n ${PORT4ME_INCLUDE_MIN} ]]; then
+        if ! grep -q -E "^[[:digit:]]+$" <<< "${PORT4ME_INCLUDE_MIN}"; then
+            _p4m_error "PORT4ME_INCLUDE_MIN is not an integer: ${PORT4ME_INCLUDE_MIN}"
+        fi
         tmp_int=${PORT4ME_INCLUDE_MIN}
         if (( tmp_int < 1 || tmp_int > 65535 )); then
             _p4m_error "PORT4ME_INCLUDE_MIN is out of range [1,65535]: ${tmp_int}"
@@ -303,6 +308,9 @@ port4me() {
     fi
 
     if [[ -n ${PORT4ME_LIST} ]]; then
+        if ! grep -q -E "^[[:digit:]]+$" <<< "${PORT4ME_LIST}"; then
+            _p4m_error "PORT4ME_LIST is not an integer: ${PORT4ME_LIST}"
+        fi
         tmp_int=${PORT4ME_LIST}
         if (( tmp_int < 1 )); then
             _p4m_error "PORT4ME_LIST must not be postive: ${tmp_int}"
@@ -310,12 +318,24 @@ port4me() {
     fi
 
     if [[ -n ${PORT4ME_SKIP} ]]; then
+        if ! grep -q -E "^[[:digit:]]+$" <<< "${PORT4ME_SKIP}"; then
+            _p4m_error "PORT4ME_SKIP is not an integer: ${PORT4ME_SKIP}"
+        fi
         tmp_int=${PORT4ME_SKIP}
         if (( tmp_int < 0 )); then
             _p4m_error "PORT4ME_SKIP must not be negative: ${tmp_int}"
         fi
     fi
     
+    mapfile -t exclude < <(_p4m_parse_ports "${PORT4ME_EXCLUDE},${PORT4ME_EXCLUDE_SITE},${PORT4ME_EXCLUDE_UNSAFE}")
+    _p4m_signal_error "${exclude[@]}"
+
+    mapfile -t include < <(_p4m_parse_ports "${PORT4ME_INCLUDE},${PORT4ME_INCLUDE_SITE}")
+    _p4m_signal_error "${include[@]}"
+    
+    mapfile -t prepend < <(_p4m_parse_ports "${PORT4ME_PREPEND},${PORT4ME_PREPEND_SITE}" false)
+    _p4m_signal_error "${prepend[@]}"
+
     if ${PORT4ME_DEBUG:-false}; then
         {
             echo "PORT4ME_EXCLUDE=${PORT4ME_EXCLUDE:-<not set>}"
diff --git a/bash/port4me b/bash/port4me
index 75054a2..db8346f 100755
--- a/bash/port4me
+++ b/bash/port4me
@@ -39,7 +39,7 @@
 #' Requirements:
 #' * Bash (>= 4)
 #'
-#' Version: 0.6.0-9014
+#' Version: 0.6.0-9015
 #' Copyright: Henrik Bengtsson (2022-2024)
 #' License: MIT
 
diff --git a/bash/tests/port4me.bats b/bash/tests/port4me.bats
index d291c18..7a2582c 100644
--- a/bash/tests/port4me.bats
+++ b/bash/tests/port4me.bats
@@ -174,3 +174,62 @@ setup() {
 @test "<CLI call> --test=<BUSY_PORT> works" {
     assert_busy_port "${cli_call[@]}"
 }
+
+
+## -------------------------------------------------------------
+## Errors
+## -------------------------------------------------------------
+@test "<CLI call> --test=65536 gives error" {
+    run "${cli_call[@]}" --test=65536
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --test=foo gives error" {
+    run "${cli_call[@]}" --test=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --list=-1 gives error" {
+    run "${cli_call[@]}" --list=-1
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --list=foo gives error" {
+    run "${cli_call[@]}" --list=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --skip=-1 gives error" {
+    run "${cli_call[@]}" --skip=-1
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --skip=foo gives error" {
+    run "${cli_call[@]}" --skip=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --exclude=foo gives error" {
+    run "${cli_call[@]}" --exclude=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --include=foo gives error" {
+    run "${cli_call[@]}" --include=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --prepend=foo gives error" {
+    run "${cli_call[@]}" --prepend=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
diff --git a/python/tests/port4me.bats b/python/tests/port4me.bats
index d291c18..7a2582c 100644
--- a/python/tests/port4me.bats
+++ b/python/tests/port4me.bats
@@ -174,3 +174,62 @@ setup() {
 @test "<CLI call> --test=<BUSY_PORT> works" {
     assert_busy_port "${cli_call[@]}"
 }
+
+
+## -------------------------------------------------------------
+## Errors
+## -------------------------------------------------------------
+@test "<CLI call> --test=65536 gives error" {
+    run "${cli_call[@]}" --test=65536
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --test=foo gives error" {
+    run "${cli_call[@]}" --test=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --list=-1 gives error" {
+    run "${cli_call[@]}" --list=-1
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --list=foo gives error" {
+    run "${cli_call[@]}" --list=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --skip=-1 gives error" {
+    run "${cli_call[@]}" --skip=-1
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --skip=foo gives error" {
+    run "${cli_call[@]}" --skip=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --exclude=foo gives error" {
+    run "${cli_call[@]}" --exclude=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --include=foo gives error" {
+    run "${cli_call[@]}" --include=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --prepend=foo gives error" {
+    run "${cli_call[@]}" --prepend=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
diff --git a/r/tests/port4me.bats b/r/tests/port4me.bats
index d291c18..7a2582c 100644
--- a/r/tests/port4me.bats
+++ b/r/tests/port4me.bats
@@ -174,3 +174,62 @@ setup() {
 @test "<CLI call> --test=<BUSY_PORT> works" {
     assert_busy_port "${cli_call[@]}"
 }
+
+
+## -------------------------------------------------------------
+## Errors
+## -------------------------------------------------------------
+@test "<CLI call> --test=65536 gives error" {
+    run "${cli_call[@]}" --test=65536
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --test=foo gives error" {
+    run "${cli_call[@]}" --test=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --list=-1 gives error" {
+    run "${cli_call[@]}" --list=-1
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --list=foo gives error" {
+    run "${cli_call[@]}" --list=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --skip=-1 gives error" {
+    run "${cli_call[@]}" --skip=-1
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --skip=foo gives error" {
+    run "${cli_call[@]}" --skip=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --exclude=foo gives error" {
+    run "${cli_call[@]}" --exclude=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --include=foo gives error" {
+    run "${cli_call[@]}" --include=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+
+@test "<CLI call> --prepend=foo gives error" {
+    run "${cli_call[@]}" --prepend=foo
+    assert_failure
+    assert_output --regexp "(error|ERROR|Error)"
+}
+