diff --git a/bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java b/bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java
index b6d082098e0fada88f2712414d224d25325187db..b13fb6030665d260ad7b4cf24b38028035619d89 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java
@@ -47,7 +47,7 @@ public class StringUtils {
 		try {
 			return s.getBytes("UTF-8");
 		} catch (UnsupportedEncodingException e) {
-			throw new RuntimeException(e);
+			throw new AssertionError(e);
 		}
 	}
 
@@ -63,7 +63,7 @@ public class StringUtils {
 		try {
 			return decoder.decode(buffer).toString();
 		} catch (CharacterCodingException e) {
-			throw new RuntimeException(e);
+			throw new AssertionError(e);
 		}
 	}
 
diff --git a/bramble-java/src/main/java/org/briarproject/bramble/plugin/modem/ModemImpl.java b/bramble-java/src/main/java/org/briarproject/bramble/plugin/modem/ModemImpl.java
index cb99c6143263f7eba531dbaf5c09d07b4165ae72..3bf2867cf6ef806eab82aba4d2a1440862cc4a1d 100644
--- a/bramble-java/src/main/java/org/briarproject/bramble/plugin/modem/ModemImpl.java
+++ b/bramble-java/src/main/java/org/briarproject/bramble/plugin/modem/ModemImpl.java
@@ -476,7 +476,7 @@ class ModemImpl implements Modem, WriteHandler, SerialPortEventListener {
 		try {
 			return decoder.decode(buffer).toString();
 		} catch (CharacterCodingException e) {
-			throw new RuntimeException(e);
+			throw new AssertionError(e);
 		}
 	}
 }