diff --git a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/FillerObjectUtil.java b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/FillerObjectUtil.java
index c790b5bf8d5d..9eb09161fa09 100644
--- a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/FillerObjectUtil.java
+++ b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/FillerObjectUtil.java
@@ -41,7 +41,6 @@
 
 import jdk.graal.compiler.api.replacements.Fold;
 import jdk.graal.compiler.core.common.NumUtil;
-import jdk.graal.compiler.word.Word;
 import jdk.vm.ci.meta.JavaKind;
 
 public class FillerObjectUtil {
@@ -50,8 +49,8 @@ public class FillerObjectUtil {
     private static final int ARRAY_ELEMENT_SIZE = ARRAY_ELEMENT_KIND.getByteCount();
 
     @Fold
-    public static UnsignedWord objectMinSize() {
-        return Word.unsigned(ConfigurationValues.getObjectLayout().getMinImageHeapObjectSize());
+    static int instanceMinSize() {
+        return ConfigurationValues.getObjectLayout().getMinRuntimeHeapInstanceSize();
     }
 
     @Fold
@@ -66,7 +65,7 @@ static int arrayBaseOffset() {
 
     @Uninterruptible(reason = CALLED_FROM_UNINTERRUPTIBLE_CODE, mayBeInlined = true)
     public static void writeFillerObjectAt(Pointer p, UnsignedWord size, boolean rememberedSet) {
-        assert size.aboveThan(0);
+        assert size.equal(instanceMinSize()) || size.aboveOrEqual(arrayMinSize());
         if (size.aboveOrEqual(arrayMinSize())) {
             int length = UnsignedUtils.safeToInt(size.subtract(arrayBaseOffset()).unsignedDivide(ARRAY_ELEMENT_SIZE));
             FormatArrayNode.formatArray(p, ARRAY_CLASS, length, rememberedSet, false, WITH_GARBAGE_IF_ASSERTIONS_ENABLED, false);
diff --git a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/TlabSupport.java b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/TlabSupport.java
index 6099dbdc6413..a22c5132d4ea 100644
--- a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/TlabSupport.java
+++ b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/TlabSupport.java
@@ -342,14 +342,16 @@ private static UnsignedWord availableTlabMemory(Descriptor tlab) {
      * If the minimum object size is greater than {@link ObjectLayout#getAlignment()}, we can end up
      * with a shard at the end of the buffer that's smaller than the smallest object (see
      * {@link com.oracle.svm.core.heap.FillerObject}). We can't allow that because the buffer must
-     * look like it's full of objects when we retire it, so we make sure we have enough space for a
-     * {@link com.oracle.svm.core.heap.FillerArray}) object.
+     * look like it's full of objects when we retire it, so we make sure we always have enough space
+     * for a filler object.
      */
     @BasedOnJDKFile("https://github.com/openjdk/jdk/blob/jdk-23-ga/src/hotspot/share/gc/shared/collectedHeap.cpp#L253-L259")
     @Uninterruptible(reason = CALLED_FROM_UNINTERRUPTIBLE_CODE, mayBeInlined = true)
     private static UnsignedWord getFillerObjectSize() {
-        UnsignedWord minSize = FillerObjectUtil.objectMinSize();
-        return minSize.aboveThan(ConfigurationValues.getObjectLayout().getAlignment()) ? minSize : Word.zero();
+        int minSize = FillerObjectUtil.instanceMinSize();
+        int alignment = ConfigurationValues.getObjectLayout().getAlignment();
+        assert FillerObjectUtil.arrayMinSize() - minSize <= alignment : "all sizes above min instance size must be fillable";
+        return (minSize > alignment) ? Word.unsigned(minSize) : Word.zero();
     }
 
     @BasedOnJDKFile("https://github.com/openjdk/jdk/blob/jdk-23-ga/src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp#L119-L124")
diff --git a/substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/config/ObjectLayout.java b/substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/config/ObjectLayout.java
index 5b9b83b2e5d8..cea1f807f402 100644
--- a/substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/config/ObjectLayout.java
+++ b/substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/config/ObjectLayout.java
@@ -273,9 +273,17 @@ public long computeArrayTotalSize(long unalignedSize, boolean withOptionalIdHash
         return alignUp(size);
     }
 
+    public int getMinRuntimeHeapInstanceSize() {
+        return getMinInstanceSize(false);
+    }
+
     public int getMinImageHeapInstanceSize() {
+        return getMinInstanceSize(true);
+    }
+
+    private int getMinInstanceSize(boolean withOptionalIdHashField) {
         int unalignedSize = firstFieldOffset; // assumes no always-present "synthetic fields"
-        if (isIdentityHashFieldAtTypeSpecificOffset() || isIdentityHashFieldOptional()) {
+        if (isIdentityHashFieldAtTypeSpecificOffset() || (withOptionalIdHashField && isIdentityHashFieldOptional())) {
             int idHashOffset = NumUtil.roundUp(unalignedSize, Integer.BYTES);
             unalignedSize = idHashOffset + Integer.BYTES;
         }