diff --git a/kuri-serde-kotlinx/src/commonMain/kotlin/org/dexpace/kuri/serde/QueryDecoder.kt b/kuri-serde-kotlinx/src/commonMain/kotlin/org/dexpace/kuri/serde/QueryDecoder.kt
index 8bf0c99..5c8f64b 100644
--- a/kuri-serde-kotlinx/src/commonMain/kotlin/org/dexpace/kuri/serde/QueryDecoder.kt
+++ b/kuri-serde-kotlinx/src/commonMain/kotlin/org/dexpace/kuri/serde/QueryDecoder.kt
@@ -56,19 +56,25 @@ internal class QueryDecoder(
 
     override fun decodeBoolean(): Boolean = parseStrictBoolean(decodeString())
 
-    override fun decodeInt(): Int = decodeString().toInt()
+    override fun decodeInt(): Int = scalarOrFail("Int", decodeString(), "value for '$currentName'", String::toIntOrNull)
 
-    override fun decodeLong(): Long = decodeString().toLong()
+    override fun decodeLong(): Long =
+        scalarOrFail("Long", decodeString(), "value for '$currentName'", String::toLongOrNull)
 
-    override fun decodeShort(): Short = decodeString().toShort()
+    override fun decodeShort(): Short =
+        scalarOrFail("Short", decodeString(), "value for '$currentName'", String::toShortOrNull)
 
-    override fun decodeByte(): Byte = decodeString().toByte()
+    override fun decodeByte(): Byte =
+        scalarOrFail("Byte", decodeString(), "value for '$currentName'", String::toByteOrNull)
 
-    override fun decodeDouble(): Double = decodeString().toDouble()
+    override fun decodeDouble(): Double =
+        scalarOrFail("Double", decodeString(), "value for '$currentName'", String::toDoubleOrNull)
 
-    override fun decodeFloat(): Float = decodeString().toFloat()
+    override fun decodeFloat(): Float =
+        scalarOrFail("Float", decodeString(), "value for '$currentName'", String::toFloatOrNull)
 
-    override fun decodeChar(): Char = decodeString().single()
+    override fun decodeChar(): Char =
+        scalarOrFail("Char", decodeString(), "value for '$currentName'") { it.singleOrNull() }
 
     override fun decodeEnum(enumDescriptor: SerialDescriptor): Int = enumIndex(enumDescriptor, decodeString())
 }
@@ -95,23 +101,31 @@ internal class QueryListDecoder(
 
     override fun decodeBoolean(): Boolean = parseStrictBoolean(next())
 
-    override fun decodeInt(): Int = next().toInt()
+    override fun decodeInt(): Int = scalarOrFail("Int", next(), "list element", String::toIntOrNull)
 
-    override fun decodeLong(): Long = next().toLong()
+    override fun decodeLong(): Long = scalarOrFail("Long", next(), "list element", String::toLongOrNull)
 
-    override fun decodeShort(): Short = next().toShort()
+    override fun decodeShort(): Short = scalarOrFail("Short", next(), "list element", String::toShortOrNull)
 
-    override fun decodeByte(): Byte = next().toByte()
+    override fun decodeByte(): Byte = scalarOrFail("Byte", next(), "list element", String::toByteOrNull)
 
-    override fun decodeDouble(): Double = next().toDouble()
+    override fun decodeDouble(): Double = scalarOrFail("Double", next(), "list element", String::toDoubleOrNull)
 
-    override fun decodeFloat(): Float = next().toFloat()
+    override fun decodeFloat(): Float = scalarOrFail("Float", next(), "list element", String::toFloatOrNull)
 
-    override fun decodeChar(): Char = next().single()
+    override fun decodeChar(): Char = scalarOrFail("Char", next(), "list element") { it.singleOrNull() }
 
     override fun decodeEnum(enumDescriptor: SerialDescriptor): Int = enumIndex(enumDescriptor, next())
 }
 
+/** Converts [raw] with [convert], failing with a [SerializationException] describing [kind] and [context] on error. */
+private fun <T : Any> scalarOrFail(
+    kind: String,
+    raw: String,
+    context: String,
+    convert: (String) -> T?,
+): T = convert(raw) ?: throw SerializationException("invalid $kind $context: '$raw'")
+
 /**
  * Parses a query value as a boolean, accepting only `"true"`/`"false"` case-insensitively.
  *
diff --git a/kuri-serde-kotlinx/src/commonTest/kotlin/org/dexpace/kuri/serde/SerdeTest.kt b/kuri-serde-kotlinx/src/commonTest/kotlin/org/dexpace/kuri/serde/SerdeTest.kt
index 3e51e40..b22d325 100644
--- a/kuri-serde-kotlinx/src/commonTest/kotlin/org/dexpace/kuri/serde/SerdeTest.kt
+++ b/kuri-serde-kotlinx/src/commonTest/kotlin/org/dexpace/kuri/serde/SerdeTest.kt
@@ -296,6 +296,99 @@ class SerdeTest {
         }
     }
 
+    @Test
+    fun `decoding a malformed Int value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<Search>("q=x&page=abc")
+        }
+    }
+
+    @Test
+    fun `decoding an empty Int value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<Search>("q=x&page=")
+        }
+    }
+
+    @Test
+    fun `decoding a malformed Long value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=1&shortValue=1&intValue=1&longValue=notanumber" +
+                    "&floatValue=1.0&doubleValue=1.0&charValue=a&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding a malformed Short value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=1&shortValue=notanumber&intValue=1&longValue=1" +
+                    "&floatValue=1.0&doubleValue=1.0&charValue=a&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding a malformed Byte value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=notanumber&shortValue=1&intValue=1&longValue=1" +
+                    "&floatValue=1.0&doubleValue=1.0&charValue=a&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding a malformed Double value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=1&shortValue=1&intValue=1&longValue=1" +
+                    "&floatValue=1.0&doubleValue=notanumber&charValue=a&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding a malformed Float value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=1&shortValue=1&intValue=1&longValue=1" +
+                    "&floatValue=notanumber&doubleValue=1.0&charValue=a&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding an empty Char value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=1&shortValue=1&intValue=1&longValue=1" +
+                    "&floatValue=1.0&doubleValue=1.0&charValue=&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding a multi-character Char value throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllScalars>(
+                "text=t&flag=true&byteValue=1&shortValue=1&intValue=1&longValue=1" +
+                    "&floatValue=1.0&doubleValue=1.0&charValue=ab&sort=ASC",
+            )
+        }
+    }
+
+    @Test
+    fun `decoding a malformed list element throws a serialization exception`() {
+        assertFailsWith<SerializationException> {
+            QueryParametersFormat.decodeFromQueryString<AllLists>(
+                "bytes=abc&shorts=1&ints=1&longs=1&floats=1.0&doubles=1.0&chars=a&flags=true&sorts=ASC",
+            )
+        }
+    }
+
     @Test
     fun `decoding a nested serializable object is rejected`() {
         assertFailsWith<SerializationException> {