diff --git a/extern/filecoin-ffi b/extern/filecoin-ffi
index 951d66ae9..b6e0b35fb 160000
--- a/extern/filecoin-ffi
+++ b/extern/filecoin-ffi
@@ -1 +1 @@
-Subproject commit 951d66ae93f6f4d3e26fee8c9320333c410f8bd1
+Subproject commit b6e0b35fb49ed0fedb6a6a473b222e3b8a7d7f17
diff --git a/go.mod b/go.mod
index df0a11740..d8efe4a3d 100644
--- a/go.mod
+++ b/go.mod
@@ -34,7 +34,7 @@ require (
 	github.com/filecoin-project/go-data-transfer v1.2.7
 	github.com/filecoin-project/go-fil-commcid v0.0.0-20201016201715-d41df56b4f6a
 	github.com/filecoin-project/go-fil-markets v1.1.7
-	github.com/filecoin-project/go-jsonrpc v0.1.2
+	github.com/filecoin-project/go-jsonrpc v0.1.3
 	github.com/filecoin-project/go-multistore v0.0.3
 	github.com/filecoin-project/go-padreader v0.0.0-20200903213702-ed5fae088b20
 	github.com/filecoin-project/go-paramfetch v0.0.2-0.20200701152213-3e0f0afdc261
diff --git a/go.sum b/go.sum
index a5c9828cc..70e789928 100644
--- a/go.sum
+++ b/go.sum
@@ -279,8 +279,8 @@ github.com/filecoin-project/go-hamt-ipld/v2 v2.0.0 h1:b3UDemBYN2HNfk3KOXNuxgTTxl
 github.com/filecoin-project/go-hamt-ipld/v2 v2.0.0/go.mod h1:7aWZdaQ1b16BVoQUYR+eEvrDCGJoPLxFpDynFjYfBjI=
 github.com/filecoin-project/go-hamt-ipld/v3 v3.0.1 h1:zbzs46G7bOctkZ+JUX3xirrj0RaEsi+27dtlsgrTNBg=
 github.com/filecoin-project/go-hamt-ipld/v3 v3.0.1/go.mod h1:gXpNmr3oQx8l3o7qkGyDjJjYSRX7hp/FGOStdqrWyDI=
-github.com/filecoin-project/go-jsonrpc v0.1.2 h1:MTebUawBHLxxY9gDi1WXuGc89TWIDmsgoDqeZSk9KRw=
-github.com/filecoin-project/go-jsonrpc v0.1.2/go.mod h1:XBBpuKIMaXIIzeqzO1iucq4GvbF8CxmXRFoezRh+Cx4=
+github.com/filecoin-project/go-jsonrpc v0.1.3 h1:Ep2PQzO1t3nUlUFXWuT12h7AfC4bZM3BjwfSDlpNzaQ=
+github.com/filecoin-project/go-jsonrpc v0.1.3/go.mod h1:XBBpuKIMaXIIzeqzO1iucq4GvbF8CxmXRFoezRh+Cx4=
 github.com/filecoin-project/go-multistore v0.0.3 h1:vaRBY4YiA2UZFPK57RNuewypB8u0DzzQwqsL0XarpnI=
 github.com/filecoin-project/go-multistore v0.0.3/go.mod h1:kaNqCC4IhU4B1uyr7YWFHd23TL4KM32aChS0jNkyUvQ=
 github.com/filecoin-project/go-padreader v0.0.0-20200903213702-ed5fae088b20 h1:+/4aUeUoKr6AKfPE3mBhXA5spIV6UcKdTYDPNU2Tdmg=
diff --git a/node/impl/client/client.go b/node/impl/client/client.go
index 00c840b63..ac526ac60 100644
--- a/node/impl/client/client.go
+++ b/node/impl/client/client.go
@@ -711,6 +711,16 @@ func (a *API) ClientCalcCommP(ctx context.Context, inpath string) (*api.CommPRet
 		return nil, err
 	}
 
+	// check that the data is a car file; if it's not, retrieval won't work
+	_, _, err = car.ReadHeader(bufio.NewReader(rdr))
+	if err != nil {
+		return nil, xerrors.Errorf("not a car file: %w", err)
+	}
+
+	if _, err := rdr.Seek(0, io.SeekStart); err != nil {
+		return nil, xerrors.Errorf("seek to start: %w", err)
+	}
+
 	pieceReader, pieceSize := padreader.New(rdr, uint64(stat.Size()))
 	commP, err := ffiwrapper.GeneratePieceCIDFromFile(arbitraryProofType, pieceReader, pieceSize)