diff --git a/runtime/src/main/java/org/capnproto/InternalError.java b/runtime/src/main/java/org/capnproto/InternalError.java
deleted file mode 100644
index e11663c..0000000
--- a/runtime/src/main/java/org/capnproto/InternalError.java
+++ /dev/null
@@ -1,7 +0,0 @@
-package org.capnproto;
-
-public final class InternalError extends RuntimeException {
-    public InternalError(String message) {
-        super(message);
-    }
-}
diff --git a/runtime/src/main/java/org/capnproto/SegmentBuilder.java b/runtime/src/main/java/org/capnproto/SegmentBuilder.java
index 6ae626c..5448ca3 100644
--- a/runtime/src/main/java/org/capnproto/SegmentBuilder.java
+++ b/runtime/src/main/java/org/capnproto/SegmentBuilder.java
@@ -28,9 +28,7 @@ public final class SegmentBuilder extends SegmentReader {
        Allocate `amount` words.
      */
     public final int allocate(int amount) {
-        if (amount < 0) {
-            throw new InternalError("tried to allocate a negative number of words");
-        }
+        assert amount >= 0 : "tried to allocate a negative number of words";
 
         if (amount > this.capacity() - this.currentSize()) {
             return FAILED_ALLOCATION; // no space left;
diff --git a/runtime/src/main/java/org/capnproto/WireHelpers.java b/runtime/src/main/java/org/capnproto/WireHelpers.java
index 23c2593..e202374 100644
--- a/runtime/src/main/java/org/capnproto/WireHelpers.java
+++ b/runtime/src/main/java/org/capnproto/WireHelpers.java
@@ -181,7 +181,7 @@ final class WireHelpers {
                                               SegmentBuilder segment,
                                               int elementCount,
                                               byte elementSize) {
-        assert elementSize == FieldSize.INLINE_COMPOSITE : "Should have called initStructListPointer instead";
+        assert elementSize != FieldSize.INLINE_COMPOSITE : "Should have called initStructListPointer instead";
 
         int dataSize = FieldSize.dataBitsPerElement(elementSize);
         int pointerCount = FieldSize.pointersPerElement(elementSize);
@@ -228,7 +228,7 @@ final class WireHelpers {
     public static ListBuilder getWritableListPointer(int origRefOffset,
                                                      SegmentBuilder origSegment,
                                                      byte elementSize) {
-        assert elementSize == FieldSize.INLINE_COMPOSITE : "Use getStructList{Element,Field} for structs";
+        assert elementSize != FieldSize.INLINE_COMPOSITE : "Use getStructList{Element,Field} for structs";
 
         long origRef = WirePointer.get(origSegment.buffer, origRefOffset);
         int origRefTarget = WirePointer.target(origRefOffset, origRef);