diff --git a/projects/common/src/client/java/dan200/computercraft/client/gui/widgets/TerminalWidget.java b/projects/common/src/client/java/dan200/computercraft/client/gui/widgets/TerminalWidget.java
index f57206205..2bf9a4494 100644
--- a/projects/common/src/client/java/dan200/computercraft/client/gui/widgets/TerminalWidget.java
+++ b/projects/common/src/client/java/dan200/computercraft/client/gui/widgets/TerminalWidget.java
@@ -72,7 +72,7 @@ public class TerminalWidget extends AbstractWidget {
     @Override
     public boolean charTyped(char ch, int modifiers) {
         var terminalChar = StringUtil.unicodeToTerminal(ch);
-        if (StringUtil.isTypableChar(terminalChar)) computer.charTyped(terminalChar);
+        if (StringUtil.isTypableChar(terminalChar)) computer.charTyped((byte) terminalChar);
         return true;
     }
 
diff --git a/projects/core/src/main/java/dan200/computercraft/core/util/StringUtil.java b/projects/core/src/main/java/dan200/computercraft/core/util/StringUtil.java
index 27e443bc6..a3af9376c 100644
--- a/projects/core/src/main/java/dan200/computercraft/core/util/StringUtil.java
+++ b/projects/core/src/main/java/dan200/computercraft/core/util/StringUtil.java
@@ -18,24 +18,25 @@ public final class StringUtil {
      * Convert a Unicode character to a terminal one.
      *
      * @param chr The Unicode character.
-     * @return The terminal character.
+     * @return The terminal character. This is either in the range [0, 255] (if a valid character) or {@code -1} if
+     * it cannot be mapped to CC's charset.
      */
-    public static byte unicodeToTerminal(int chr) {
+    public static int unicodeToTerminal(int chr) {
         // ASCII and latin1 map to themselves
         if (chr == 0 || chr == '\t' || chr == '\n' || chr == '\r' || (chr >= ' ' && chr <= '~') || (chr >= 160 && chr <= 255)) {
-            return (byte) chr;
+            return chr;
         }
 
         // Teletext block mosaics are *fairly* contiguous.
-        if (chr >= 0x1FB00 && chr <= 0x1FB13) return (byte) (chr + (129 - 0x1fb00));
-        if (chr >= 0x1FB14 && chr <= 0x1FB1D) return (byte) (chr + (150 - 0x1fb14));
+        if (chr >= 0x1FB00 && chr <= 0x1FB13) return chr + (129 - 0x1fb00);
+        if (chr >= 0x1FB14 && chr <= 0x1FB1D) return chr + (150 - 0x1fb14);
 
         // Everything else is just a manual lookup. For now, we just use a big switch statement, which we spin into a
         // separate function to hopefully avoid inlining it here.
         return unicodeToCraftOsFallback(chr);
     }
 
-    private static byte unicodeToCraftOsFallback(int c) {
+    private static int unicodeToCraftOsFallback(int c) {
         return switch (c) {
             case 0x263A -> 1;
             case 0x263B -> 2;
@@ -64,8 +65,8 @@ public final class StringUtil {
             case 0x25B2 -> 30;
             case 0x25BC -> 31;
             case 0x1FB99 -> 127;
-            case 0x258C -> (byte) 149;
-            default -> '?';
+            case 0x258C -> 149;
+            default -> -1;
         };
     }
 
@@ -76,8 +77,8 @@ public final class StringUtil {
      * @param chr The character to check.
      * @return Whether this character can be typed.
      */
-    public static boolean isTypableChar(byte chr) {
-        return chr != 0 && chr != '\r' && chr != '\n';
+    public static boolean isTypableChar(int chr) {
+        return chr >= 0 && chr <= 255 && chr != 0 && chr != '\r' && chr != '\n';
     }
 
     private static boolean isAllowedInLabel(char c) {
@@ -110,8 +111,9 @@ public final class StringUtil {
         var iterator = clipboard.codePoints().iterator();
         while (iterator.hasNext() && idx <= output.length) {
             var chr = unicodeToTerminal(iterator.next());
-            if (!isTypableChar(chr)) break;
-            output[idx++] = chr;
+            if (chr < 0) continue; // Strip out unconvertible characters
+            if (!isTypableChar(chr)) break; // Stop at untypable ones.
+            output[idx++] = (byte) chr;
         }
 
         return ByteBuffer.wrap(output, 0, idx).asReadOnlyBuffer();
diff --git a/projects/standalone/src/main/java/cc/tweaked/standalone/InputState.java b/projects/standalone/src/main/java/cc/tweaked/standalone/InputState.java
index 2013a1fb3..4adbfd3a9 100644
--- a/projects/standalone/src/main/java/cc/tweaked/standalone/InputState.java
+++ b/projects/standalone/src/main/java/cc/tweaked/standalone/InputState.java
@@ -51,7 +51,7 @@ public class InputState {
 
     public void onCharEvent(int codepoint) {
         var terminalChar = StringUtil.unicodeToTerminal(codepoint);
-        if (StringUtil.isTypableChar(terminalChar)) ComputerEvents.charTyped(computer, terminalChar);
+        if (StringUtil.isTypableChar(terminalChar)) ComputerEvents.charTyped(computer, (byte) terminalChar);
     }
 
     public void onKeyEvent(long window, int key, int action, int modifiers) {