System.ArgumentException: Member 'UnityEngine.AudioClip' is declared in another module and needs to be imported

I updated my project from Unity 5.4.3 to Unity 5.6.0. It runs fine in the editor, but when I try to build iOS, I receive this error:

 Failed running /Applications/Unity 5.6.0/Unity 5.6.0.app/Contents/Tools/UnusedByteCodeStripper2/UnusedBytecodeStripper2.exe...
 
 Fatal error in Mono CIL Linker
 System.ArgumentException: Member 'UnityEngine.AudioClip' is declared in another module and needs to be imported
   at Mono.Cecil.MetadataBuilder.LookupToken (Mono.Cecil.IMetadataTokenProvider provider) [0x00027] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.SignatureWriter.MakeTypeDefOrRefCodedRID (Mono.Cecil.TypeReference type) [0x00000] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.SignatureWriter.WriteTypeSignature (Mono.Cecil.TypeReference type) [0x00161] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.SignatureWriter.WriteMethodSignature (Mono.Cecil.IMethodSignature method) [0x00085] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.GetMethodSignature (Mono.Cecil.IMethodSignature method) [0x00006] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.AddMethod (Mono.Cecil.MethodDefinition method) [0x0003c] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.AddMethods (Mono.Cecil.TypeDefinition type) [0x00013] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.AddType (Mono.Cecil.TypeDefinition type) [0x000a2] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.AddTypeDefs () [0x00018] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.BuildTypes () [0x00014] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.BuildModule () [0x0007e] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.MetadataBuilder.BuildMetadata () [0x00000] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.ModuleWriter+<>c.<BuildMetadata>b__1_0 (Mono.Cecil.MetadataBuilder builder, Mono.Cecil.MetadataReader _) [0x00000] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.ModuleDefinition.Read[TItem,TRet] (TItem item, System.Func`3[T1,T2,TResult] read) [0x00029] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.ModuleWriter.BuildMetadata (Mono.Cecil.ModuleDefinition module, Mono.Cecil.MetadataBuilder metadata) [0x0000f] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.ModuleWriter.WriteModuleTo (Mono.Cecil.ModuleDefinition module, System.IO.Stream stream, Mono.Cecil.WriterParameters parameters) [0x000b7] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.ModuleDefinition.Write (System.IO.Stream stream, Mono.Cecil.WriterParameters parameters) [0x00022] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.ModuleDefinition.Write (System.String fileName, Mono.Cecil.WriterParameters parameters) [0x0000a] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Cecil.AssemblyDefinition.Write (System.String fileName, Mono.Cecil.WriterParameters parameters) [0x00000] in <0304696c64424476882bed0a697f98eb>:0 
   at Mono.Linker.Steps.OutputStep.OutputAssembly (Mono.Cecil.AssemblyDefinition assembly) [0x0004d] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 
   at Mono.Linker.Steps.OutputStep.ProcessAssembly (Mono.Cecil.AssemblyDefinition assembly) [0x00001] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 
   at Mono.Linker.Steps.BaseStep.Process (Mono.Linker.LinkContext context) [0x00031] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 
   at Mono.Linker.Pipeline.Process (Mono.Linker.LinkContext context) [0x00019] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 
   at Mono.Linker.Driver.Run (System.Collections.Generic.IEqualityComparer`1[T] typeComparer, System.Collections.Generic.IEqualityComparer`1[T] methodComparer) [0x003e2] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 
   at Mono.Linker.Driver.RunDriverWithErrorHandling (Mono.Linker.Driver driver, System.Collections.Generic.IEqualityComparer`1[T] typeComparer, System.Collections.Generic.IEqualityComparer`1[T] methodComparer) [0x00001] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 
   at Mono.Linker.Driver.RunDriver (Mono.Linker.Driver driver, System.Collections.Generic.IEqualityComparer`1[T] typeComparer, System.Collections.Generic.IEqualityComparer`1[T] methodComparer) [0x00002] in <6b6a6b3d682b49c4a8b0c4f50e76a8a6>:0 

This seems like the crucial error statement:

 System.ArgumentException: Member 'UnityEngine.AudioClip' is declared in another module and needs to be imported

But AudioClip should be inherent and doesn't need an import statement...

Everything worked great before I updated. Any suggestions?